The nakedness of a woman and her sister: To not have intercourse with two sisters, meaning to say that a man not marry two sisters together — and not even this one after that one — during the lifetime of the first one; and even if he divorced the first one, as it is stated (Leviticus 18:18), “And a woman upon her sister shall you not take to be a rival, to reveal her nakedness upon her during her life.” The explanation is a woman and her sister together, “shall you not take to be a rival,” [which] is an expression of rivalry — meaning to say that he should not make one the rival of her friend; “during her life” comes to teach that even [if] he divorced the first, he shall not marry her sister — and that is [the meaning of] the expression, “during her life,” meaning to say all the time that she is alive. But after the death of one, there is no doubt that it is permitted to marry the other. And that is what the verse stated, “during her life.” And likewise, if he lays in the way of licentiousness with the sister after he married the first one or designated her, it is also included in the prohibition; as the Torah was concerned about the marriage of [only] the first one.
The matter of the sexual prohibitions is from the roots of the commandment, [about which] we wrote that which we knew above (Sefer HaChinukh 190). And also my heart tells me about this matter that Scripture forbade to marry two sisters, that the Master of peace wants peace [for] all of His creatures and, all the more so, those creatures that nature and intellect obligate to have peace between them — and not constant quarrel and competition the whole day.
From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 54b) that she is called the sister of his wife, whether she is the sister from her mother or the sister from her father; whether from marriage or whether from licentiousness. And they also explained that the prohibition comes into force from when he designates her, and as the expression of taking of a woman is always understood by us, as we said. And also that which they, may their memory be blessed, forbade the sister of his bound one, as a fence for this prohibition. And the understanding of his bound one is such a case as one who has a brother and he dies and does not leave children, such that he is obligated by the Torah to marry his [brother’s] wife or to release her, as it appears explicitly in the written commandment (Sefer HaChinukh 597, 599). And that is the woman that is bound to the brother, meaning to say that she is in the domain of the brother to marry her if he wants. Since she is acquired by him from the Heavens, she is called, “his bound one.” And they, may their memory be blessed, forbade (Yevamot 41a) him from marrying her sister from when she is bound to him, even though he did not designate her and did not do any action.
And likewise they also forbade him (Yevamot 40a) as a fence for this, the sister of his released one (chalitsato); meaning to say, they forbade him from marrying her sister, even after he released her. Just like Scripture forbade the sister of his divorcee, as we explained; so too did [the Sages] also forbid the sister of his released one — since release is similar to a bill of divorce. And nonetheless, it is established for us on the legal level that the prohibition of the sister of his released one — and even the sister of his bound one - is rabbinic. As the Torah only forbade the sister of a woman that he married or betrothed. But the sister of a woman bound to him — even if [the latter] was acquired by him from the Heavens — is not included in the Torah prohibition. Rather, [it is] they, may their memory be blessed, that prohibited her, so as not to touch the Torah prohibition. And since this prohibition is [only] rabbinic, as we said, they were sometimes a little lenient in that matter — to the point that they, may their memory be blessed, said (Yevamot 41a) that we say to one of the brothers required for levirate marriage who designated the sister of [this] levirate wife, “Wait. Do not divorce her and do not marry her until your brother performs levirate marriage or releases this levirate wife that is bound to all of you.” [If] his brother released her or performed levirate marriage, or [if] the levirate wife dies, he can [marry] his betrothed. But if all of the [other] brothers die, he divorces his betrothed with a bill of divorce, and his levirate wife with release (chalitsah). And if his betrothed dies — whether she died before the death of the brothers or whether she died after their death — the levirate wife comes back to be permitted [to him]. If he wants, he releases [her]; if he wants, he performs levirate marriage. And in the commandment of levirate marriage, we will write a little further about these matters (Sefer HaChinukh 498) — as is our custom — with God’s help; as here is not its place.
And this warning (negative commandment) is practiced in every place and at all times. And one who transgresses it and volitionally laid with the sister — whether by way of marriage, or by way of licentiousness — during the lifetime of the first, is liable for excision; and he is lashed, if there are witnesses and a warning; inadvertently, he brings a sin-offering. And if he laid with the sister of his bound one or the sister of his released one, he is struck by rabbinic lashes of rebellion. And nonetheless, his wife — that is the sister that he married first — is not forbidden to him, because of the marriage to the second or (her licentiousness) because he was licentious with her. Rather, his wife is permitted to him, and this second one should go away (Yevamot 94a). And even if he married [the second] with “a canopy and designation,” she does not require a bill of divorce from him — as designation is not effective with sexual prohibitions, to have her require a bill of divorce. But nonetheless one who does an act of designation (kiddushin) with them makes himself liable, as the Torah stated, “you shall not take” — which implies a language of designation. And even though if he transgressed and took, meaning designated, [her], it does not help him — meaning, she is not “taken” by him for any matter, and [so] she does not require a bill of divorce from him — nonetheless, he becomes liable with this, as we said.
And this law is not only with sisters, but rather also with all the other sexual prohibitions: that if he marries them with the assumption [that they are] permitted and they turn out to be sexual prohibitions, they do not require a bill of divorce from the reason that we said — that the designation is not effective with sexual prohibitions. And likewise, if he married or transgressed and was adulterous with one of the six women that we said above (Sefer HaChinukh 205) are sexual prohibitions for him — on account of his wife — from Torah writ, they do not prohibit his wife to him; but rather they should go away without a bill of divorce. And his wife remains permitted to him, as we said. [This is the case] except with the sister of his betrothed, wherewith the Sages were stringent and forbade even the first one, because of the second one. As they, may their memory be blessed, said (Yevamot 94b) that if a man betroths a woman — meaning to say, he designates her — and afterwards marries her sister, both of them are forbidden to him and [both] require a bill of divorce from him (Mishneh Torah, Laws of Divorce 10:9, 10). And because of what did they say this, and they did not say that he [stays] married to the first one and the second one goes away without a bill of divorce, as we wrote above? Because the Sages were concerned, lest the creatures say since there was only designation alone with the first one, that there was some condition in that designation — and as a result, he married the second; and he married her legally. And therefore, they required a bill of divorce for her. And since they required a bill of divorce for the later one — because of the concern that we said — they needed to also forbid his first betrothed; so that the creatures not say, he married the sister of the woman he divorced.
עֶרְוַת אִשָּׁה וַאֲחוֹתָהּ – שֶׁלֹּא לָבוֹא עַל שְׁתֵּי אֲחָיוֹת. כְּלוֹמַר, שֶׁלֹּא יִשָּׂא אָדָם שְׁתֵּי אֲחָיוֹת בְּיַחַד וְלֹא אֲפִלּוּ זוֹ אַחַר זוֹ בְּחַיֵּי הָרִאשׁוֹנָה וַאֲפִלּוּ גֵּרְשָׁהּ לָרִאשׁוֹנָה שֶׁנֶּאֱמַר (ויקרא יח יח) וְאִשָּׁה אֶל אֲחֹתָהּ לֹא תִקָּח לִצְרֹר לְגַלּוֹת עֶרְוָתָהּ עָלֶיהָ בְּחַיֶּיהָ. פֵּרוּשׁ, וְאִשָּׁה עִם אֲחוֹתָהּ בְּיַחַד, לֹא תִּקַּח, לִצְרֹר לְשׁוֹן צָרָה, כְּלוֹמַר שֶׁלֹּא תֵּעָשֶׂה הָאַחַת צָרָה לַחֲבֶרְתָּהּ. בְּחַיֶּיהָ, בָּא וְלִמֵּד שֶׁאֲפִלּוּ גֵּרְשָׁהּ לָרִאשׁוֹנָה לֹא יִשָּׂא אֲחוֹתָהּ, וְזֶהוּ לְשׁוֹן בְּחַיֶּיהָ כְּלוֹמַר, כָּל זְמַן שֶׁהִיא בַחַיִּים אֲבָל אַחַר מוֹת הָאַחַת אֵין סָפֵק שֶׁמֻּתָּר לִשָּׂא הָאַחֶרֶת, וְהַיְנוּ מַה שֶּׁאָמַר הַכָּתוּב בְּחַיֶּיהָ. וְכֵן הַשּׁוֹכֵב דֶּרֶךְ זְנוּת עִם הָאָחוֹת אַחַר שֶׁנָּשָׂא הָרִאשׁוֹנָה אוֹ קִדְּשָׁהּ, גַּם כֵּן בִּכְלַל הָאִסּוּר, כִּי בְּנִשּׂוּאֵי הָרִאשׁוֹנָה הִקְפִּידָה תּוֹרָה (יבמות צז א).
The nakedness of a woman and her sister: To not have intercourse with two sisters, meaning to say that a man not marry two sisters together — and not even this one after that one — during the lifetime of the first one; and even if he divorced the first one, as it is stated (Leviticus 18:18), “And a woman upon her sister shall you not take to be a rival, to reveal her nakedness upon her during her life.” The explanation is a woman and her sister together, “shall you not take to be a rival,” [which] is an expression of rivalry — meaning to say that he should not make one the rival of her friend; “during her life” comes to teach that even [if] he divorced the first, he shall not marry her sister — and that is [the meaning of] the expression, “during her life,” meaning to say all the time that she is alive. But after the death of one, there is no doubt that it is permitted to marry the other. And that is what the verse stated, “during her life.” And likewise, if he lays in the way of licentiousness with the sister after he married the first one or designated her, it is also included in the prohibition; as the Torah was concerned about the marriage of [only] the first one.
מִשָּׁרְשֵׁי הַמִּצְוָה, עִנְיַן הָעֲרָיוֹת כָּתַבְנוּ לְמַעְלָה (מצוה קצ) מָה שֶׁיָּדַעְנוּ. וְעוֹד אוֹמֵר לִי לִבִּי בְּעִנְיָן זֶה שֶׁאָסַר הַכָּתוּב לִשָּׂא שְׁתֵּי אֲחָיוֹת, כִּי אֲדוֹן הַשָּׁלוֹם יַחְפֹּץ בִּשְׁלוֹם כָּל בְּרִיּוֹתָיו, וְכָל שֶׁכֵּן בִּשְׁלוֹם אוֹתָן בְּרִיּוֹת שֶׁהַטֶּבַע וְהַשֵּׂכֶל מְחַיְּבִין לִהְיוֹת שָׁלוֹם בֵּינֵיהֶם וְלֹא קְטָטָה וְתַחֲרוּת תָּמִיד כָּל הַיּוֹם.
The matter of the sexual prohibitions is from the roots of the commandment, [about which] we wrote that which we knew above (Sefer HaChinukh 190). And also my heart tells me about this matter that Scripture forbade to marry two sisters, that the Master of peace wants peace [for] all of His creatures and, all the more so, those creatures that nature and intellect obligate to have peace between them — and not constant quarrel and competition the whole day.
מִדִּינֵי הַמִּצְוָה. מָה שֶׁפֵּרְשׁוּ זִכְרוֹנָם לִבְרָכָה (יבמות נד, ב) שֶׁאֲחוֹת אִשְׁתּוֹ נִקְרֵאת בֵּין אֲחוֹתָהּ מֵאִמָּהּ בֵּין אֲחוֹתָהּ מֵאָבִיהָ, בֵּין מִן הַנִּשּׂוּאִין, בֵּין מִן הַזְּנוּת. וּפֵרְשׁוּ גַּם כֵּן, שֶׁהָאִסּוּר יָחוּל מִכֵּיוָן שֶׁיְּקַדְּשֶׁנָּה, וְכִדְמַשְׁמַע לַן לְשׁוֹן קִיחָה בְּאִשָּׁה לְעוֹלָם, כְּמוֹ שֶׁאָמַרְנוּ. וְכֵן מָה שֶׁאָסְרוּ זִכְרוֹנָם לִבְרָכָה לְגֶדֶר אִסּוּר זֶה אֲחוֹת זְקוּקָתוֹ, פֵּרוּשׁ זְקוּקָתוֹ כְּגוֹן מִי שֶׁהָיָה לוֹ אָח וּמֵת וְלֹא הִנִּיחַ בָּנִים, שֶׁהוּא חַיָּב לִשָּׂא אִשְׁתּוֹ מִן הַתּוֹרָה אוֹ לַחֲלֹץ לָהּ, כְּמוֹ שֶׁבָּא הַמִּצְוָה בַּכָּתוּב בְּפֵרוּשׁ (מצוה תקצז תקצט) וְזֹאת הָאִשָּׁה שֶׁהִיא זְקוּקָה לְאָח, כְּלוֹמַר, שֶׁהִיא בִּרְשׁוּת הָאָח לִשָּׂא אוֹתָהּ אִם יִרְצֶה, כִּי מִן הַשָּׁמַיִם הִקְנוּהָ לוֹ, תִּקָּרֵא זְקוּקָתוֹ. וְאָסְרוּ זִכְרוֹנָם לִבְרָכָה (שם מא, א) מִלִּשָּׂא אֲחוֹתָהּ, מִכֵּיוָן שֶׁהִיא זְקוּקָה אֵלָיו, אַף עַל פִּי שֶׁלֹּא קִדְּשָׁהּ וְלֹא עָשָׂה בָּהּ שׁוּם עִנְיָן.
From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 54b) that she is called the sister of his wife, whether she is the sister from her mother or the sister from her father; whether from marriage or whether from licentiousness. And they also explained that the prohibition comes into force from when he designates her, and as the expression of taking of a woman is always understood by us, as we said. And also that which they, may their memory be blessed, forbade the sister of his bound one, as a fence for this prohibition. And the understanding of his bound one is such a case as one who has a brother and he dies and does not leave children, such that he is obligated by the Torah to marry his [brother’s] wife or to release her, as it appears explicitly in the written commandment (Sefer HaChinukh 597, 599). And that is the woman that is bound to the brother, meaning to say that she is in the domain of the brother to marry her if he wants. Since she is acquired by him from the Heavens, she is called, “his bound one.” And they, may their memory be blessed, forbade (Yevamot 41a) him from marrying her sister from when she is bound to him, even though he did not designate her and did not do any action.
וְכֵן אָסְרוּ עָלָיו (שם מ, א) גַּם כֵּן לְגֶדֶר זֶה אֲחוֹת חֲלוּצָתוֹ, כְּלוֹמַר, אֲפִלּוּ אַחַר שֶׁחָלַץ לָהּ, אָסְרוּ עָלָיו מִלִּשָּׂא אֲחוֹתָהּ, כְּמוֹ שֶׁאָסַר הַכָּתוּב אֲחוֹת גְּרוּשָׁתוֹ, כְּמוֹ שֶׁפֵּרַשְׁנוּ, כֵּן אָסְרוּ גַּם כֵּן אֲחוֹת חֲלוּצָתוֹ, שֶׁהַחֲלִיצָה כְּעֵין גֵּט הִיא. וּמִכָּל מָקוֹם אִסּוּר אֲחוֹת חֲלוּצָתוֹ וַאֲפִילּוּ אֲחוֹת זְקוּקָתוֹ קַיְמָא לַן אַלִּבָּא דְּהִלְכְתָא דְּמִדְּרַבָּנָן הוּא, כִּי הַתּוֹרָה לֹא אָסְרָה כִּי אִם אֲחוֹת נְשׂוּאָתוֹ אוֹ אֲרוּסָתוֹ אֲבָל הַזְּקוּקָה לוֹ, אַף עַל פִּי שֶׁמִּן הַשָּׁמַיִם הִקְנוּהָ לוֹ אֵין אֲחוֹתָהּ בִּכְלַל אִסּוּר תּוֹרָה, אֶלָּא שֶׁהֵם זִכְרוֹנָם לִבְרָכָה אֲסָרוּהָ לוֹ שֶׁלֹּא לִגַּע לְאִסּוּרָא דְּאוֹרָיְתָא. וּמִפְּנֵי שֶׁאִסּוּר זֶה הוּא מִדִּבְרֵיהֶם, כְּמוֹ שֶׁאָמַרְנוּ, הֵקֵלּוּ בָּעִנְיָן קְצָת לִפְעָמִים, עַד שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה (שם מא, א) כִּי אֶחָד מִן הַיְּבָמִין שֶׁקִּדֵּשׁ אֲחוֹת יְבִמְתּוֹ שֶׁאוֹמְרִים לוֹ, הַמְתֵּן אַל תְּגָרְשֶׁנָּה וְאַל תִּשָּׂאֶנָּה עַד שֶׁיְּיַבֵּם אָחִיךָ אוֹ יַחֲלֹץ לִיבָמָה זוֹ הַזְּקוּקָה לְכֻלְּכֶם. חָלַץ לָהּ אָחִיו אוֹ יִבְּמָהּ אוֹ שֶׁמֵּתָה הַיְּבָמָה הֲרֵי זֶה יִכְנֹס אֲרוּסָתוֹ. אֲבָל אִם מֵתוּ הָאַחִים כֻּלָּם מוֹצִיא אֶת אֲרוּסָתוֹ בְּגֵט וִיבִמְתּוֹ בַּחֲלִיצָה. וְאִם מֵתָה אֲרוּסָתוֹ, בֵּין שֶׁמֵּתָה קֹדֶם מִיתַת הָאַחִין בֵּין שֶׁמֵּתָה לְאַחַר מִיתָתָם, חָזְרָה הַיְּבָמָה לְהֶתֵּרָהּ, רָצָה חוֹלֵץ רָצָה מְיַבֵּם. וּבְמִצְוַת יִבּוּם נִכְתֹּב עוֹד מְעַט בְּעִנְיָנִים אֵלֶּה (מצוה תקצח), כְּמִנְהָגֵנוּ בְּעֶזְרַת הַשֵּׁם, שֶׁאֵין כָּאן מְקוֹמָן.
And likewise they also forbade him (Yevamot 40a) as a fence for this, the sister of his released one (chalitsato); meaning to say, they forbade him from marrying her sister, even after he released her. Just like Scripture forbade the sister of his divorcee, as we explained; so too did [the Sages] also forbid the sister of his released one — since release is similar to a bill of divorce. And nonetheless, it is established for us on the legal level that the prohibition of the sister of his released one — and even the sister of his bound one - is rabbinic. As the Torah only forbade the sister of a woman that he married or betrothed. But the sister of a woman bound to him — even if [the latter] was acquired by him from the Heavens — is not included in the Torah prohibition. Rather, [it is] they, may their memory be blessed, that prohibited her, so as not to touch the Torah prohibition. And since this prohibition is [only] rabbinic, as we said, they were sometimes a little lenient in that matter — to the point that they, may their memory be blessed, said (Yevamot 41a) that we say to one of the brothers required for levirate marriage who designated the sister of [this] levirate wife, “Wait. Do not divorce her and do not marry her until your brother performs levirate marriage or releases this levirate wife that is bound to all of you.” [If] his brother released her or performed levirate marriage, or [if] the levirate wife dies, he can [marry] his betrothed. But if all of the [other] brothers die, he divorces his betrothed with a bill of divorce, and his levirate wife with release (chalitsah). And if his betrothed dies — whether she died before the death of the brothers or whether she died after their death — the levirate wife comes back to be permitted [to him]. If he wants, he releases [her]; if he wants, he performs levirate marriage. And in the commandment of levirate marriage, we will write a little further about these matters (Sefer HaChinukh 498) — as is our custom — with God’s help; as here is not its place.
וְנוֹהֶגֶת אַזְהָרָה זוֹ בְּכָל מָקוֹם וּבְכָל זְמַן. וְהָעוֹבֵר עָלֶיהָ וְשָׁכַב עִם הָאָחוֹת, בֵּין עַל יְדֵי נִשּׂוּאִין, בֵּין דֶּרֶךְ זְנוּת, בְּחַיֵּי הָרִאשׁוֹנָה בְּמֵזִיד חַיָּב כָּרֵת, וְאִם יֵשׁ עֵדִים וְהַתְרָאָה לוֹקֶה, בְּשׁוֹגֵג מֵבִיא חַטָּאת קְבוּעָה. וְאִם שָׁכַב עִם אֲחוֹת זְקוּקָתוֹ אוֹ אֲחוֹת חֲלוּצָתוֹ לוֹקֶה מַכַּת מַרְדּוּת דְּרַבָּנָן. וּמִכָּל מָקוֹם, אֵין אִשְׁתּוֹ דְּהַיְנוּ הָאָחוֹת שֶׁנָּשָׂא רִאשׁוֹנָה אֲסוּרָה עָלָיו מִפְּנֵי נִשּׂוּאֵי שְׁנִיָּה אוֹ (זִמָּתָהּ) מִפְּנֵי שֶׁזָּנְתָה, אֶלָּא אִשְׁתּוֹ מֻתֶּרֶת לוֹ, וְזֹאת הַשֵּׁנִית תֵּלֵךְ לָהּ (יבמות צד א). וַאֲפִלּוּ נְשָׂאָהּ עַל יְדֵי חֻפָּה וְקִדּוּשִׁין אֵינָהּ צְרִיכָה מִמֶּנּוּ גֵט, שֶׁאֵין קִדּוּשִׁין תּוֹפְסִין בָּעֲרָיוֹת לְהַצְרִיכָן גֵּט, אֲבָל מִכָּל מָקוֹם מִתְחַיֵּב הָעוֹשֶׂה בָּהֶן מַעֲשֵׂה הַקִּדּוּשִׁין, שֶׁהַתּוֹרָה אָמְרָה לֹא תִקַּח, וּמַשְׁמַע לְשׁוֹן קִדּוּשִׁין, וְאַף עַל גַּב דְּאִם עָבַר וְלָקַח, כְּלוֹמַר, שֶׁקִּדֵּשׁ, לָא מְהַנֵּי לֵיהּ כְּלוֹמַר, שֶׁאֵינָהּ לְקוּחָתוֹ לְשׁוּם דָּבָר וְאֵינָהּ צְרִיכָה מִמֶּנּוּ גֵט, מִכָּל מָקוֹם נִתְחַיֵּב בְּכָךְ כְּמוֹ שֶׁאָמַרְנוּ.
And this warning (negative commandment) is practiced in every place and at all times. And one who transgresses it and volitionally laid with the sister — whether by way of marriage, or by way of licentiousness — during the lifetime of the first, is liable for excision; and he is lashed, if there are witnesses and a warning; inadvertently, he brings a sin-offering. And if he laid with the sister of his bound one or the sister of his released one, he is struck by rabbinic lashes of rebellion. And nonetheless, his wife — that is the sister that he married first — is not forbidden to him, because of the marriage to the second or (her licentiousness) because he was licentious with her. Rather, his wife is permitted to him, and this second one should go away (Yevamot 94a). And even if he married [the second] with “a canopy and designation,” she does not require a bill of divorce from him — as designation is not effective with sexual prohibitions, to have her require a bill of divorce. But nonetheless one who does an act of designation (kiddushin) with them makes himself liable, as the Torah stated, “you shall not take” — which implies a language of designation. And even though if he transgressed and took, meaning designated, [her], it does not help him — meaning, she is not “taken” by him for any matter, and [so] she does not require a bill of divorce from him — nonetheless, he becomes liable with this, as we said.
וְזֶה הַדִּין אֵינוֹ בַּאֲחָיוֹת לְבַד, אֶלָּא אַף בְּכָל שְׁאָר הָעֲרָיוֹת הוּא, שֶׁאִם נְשָׂאָן בְּחֶזְקַת הֶתֵּר וְנִמְצְאוּ עֲרָיוֹת שֶׁאֵינָן צְרִיכוֹת גֵּט מִן הַטַּעַם שֶׁאָמַרְנוּ, שֶׁאֵין קִדּוּשִׁין תּוֹפְסִין בָּעֲרָיוֹת. וְכֵן אִם נָשָׂא אוֹ עָבַר וְנָאַף עִם אַחַת מִן הַשֵּׁשׁ נָשִׁים שֶׁאָמַרְנוּ לְמַעְלָה (מצוה רב) שֶׁהֵן עֲרָיוֹת עָלָיו מִן הַתּוֹרָה מֵחֲמַת אִשְׁתּוֹ, אֵינָן אוֹסְרוֹת אִשְׁתּוֹ עָלָיו אֶלָּא יֵלְכוּ לָהֶן בְּלֹא גֵּט וְתִשָּׁאֵר אִשְׁתּוֹ מֻתֶּרֶת לוֹ, כְּמוֹ שֶׁאָמַרְנוּ, זוּלָתִי בַּאֲחוֹת אֲרוּסָתוֹ שֶׁהֶחְמִירוּ חֲכָמִים וְאָסְרוּ אַף הָרִאשׁוֹנָה מִפְּנֵי הַשְּׁנִיָּה, שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה (שם צד, ב) שֶׁאִם אֵרַס אָדָם אִשָּׁה, כְּלוֹמַר, שֶׁקִּדְּשָׁהּ וְאַחַר כָּךְ נָשָׂא אֲחוֹתָהּ, שֶׁשְּׁתֵּיהֶן אֲסוּרוֹת עָלָיו וּצְרִיכוֹת מִמֶּנּוּ גֵּט (עי' רמב"ם גירושין פ"י הל' ט, י). וּמִפְּנֵי מָה אָמְרוּ כֵּן וְלֹא אָמְרוּ שֶׁיִּשָּׂא הָרִאשׁוֹנָה וְהַשֵּׁנִית תֵּלֵךְ לָהּ בְּלֹא גֵּט כְּמוֹ שֶׁכָּתוּב לְמַעְלָה, לְפִי שֶׁחָשׁוּ חֲכָמִים שֶׁמָּא יֹאמְרוּ הַבְּרִיּוֹת אַחַר שֶׁלֹּא הָיָה בָּרִאשׁוֹנָה אֶלָּא קִדּוּשִׁין לְבַד, כִּי בְּאוֹתָן קִדּוּשִׁין הָיָה שׁוּם תְּנַאי, וְעַל כֵּן נָשָׂא הַשֵּׁנִית וּבְדִין נְשָׂאָהּ, וּלְפִיכָךְ הִצְרִיכוּ בָּהּ גֵּט, וְאַחַר שֶׁהִצְרִיכוּ בָּאַחֲרוֹנָה גֵּט מִפְּנֵי זֶה הַחֲשָׁשׁ שֶׁאָמַרְנוּ, הָיוּ צְרִיכִים לֶאֱסֹר גַּם כֵּן אֲרוּסָתוֹ הָרִאשׁוֹנָה כְּדֵי שֶׁלֹּא יֹאמְרוּ הַבְּרִיּוֹת נָשָׂא אֲחוֹת גְּרוּשָׁתוֹ.
And this law is not only with sisters, but rather also with all the other sexual prohibitions: that if he marries them with the assumption [that they are] permitted and they turn out to be sexual prohibitions, they do not require a bill of divorce from the reason that we said — that the designation is not effective with sexual prohibitions. And likewise, if he married or transgressed and was adulterous with one of the six women that we said above (Sefer HaChinukh 205) are sexual prohibitions for him — on account of his wife — from Torah writ, they do not prohibit his wife to him; but rather they should go away without a bill of divorce. And his wife remains permitted to him, as we said. [This is the case] except with the sister of his betrothed, wherewith the Sages were stringent and forbade even the first one, because of the second one. As they, may their memory be blessed, said (Yevamot 94b) that if a man betroths a woman — meaning to say, he designates her — and afterwards marries her sister, both of them are forbidden to him and [both] require a bill of divorce from him (Mishneh Torah, Laws of Divorce 10:9, 10). And because of what did they say this, and they did not say that he [stays] married to the first one and the second one goes away without a bill of divorce, as we wrote above? Because the Sages were concerned, lest the creatures say since there was only designation alone with the first one, that there was some condition in that designation — and as a result, he married the second; and he married her legally. And therefore, they required a bill of divorce for her. And since they required a bill of divorce for the later one — because of the concern that we said — they needed to also forbid his first betrothed; so that the creatures not say, he married the sister of the woman he divorced.