The nakedness of your daughter-in-law you shall not reveal: To not have intercourse with the wife of the son, which is his wife from the betrothal — and all the more so from the marriage — whether during his lifetime and even after he divorced her, or whether after his death. In any manner [above], she is called the wife of his son and she is forbidden to him, as it is stated (Leviticus 18:15), “The nakedness of your daughter-in-law you shall not reveal.” And from that which appears, the liability is even with a son that is a mamzer — he is his son regardless. But the son from a maidservant or from a gentile is not called “his son” for anything.
From the laws of the commandment is, for example, that which they, may their memory be blessed, forbade (Yevamot 21b) — to make a fence for this — the daughter-in-law of his son and likewise the daughter-in-law of the son of his son, to the end of the world. And they also forbade only the daughter-in-law of his daughter. And the reason is similar to that which we said above — since the prohibition came in the verse about his daughter-in-law, they decreed about all daughters-in-law that come on account of the son; and they decreed with [the daughter-in-law of] the daughter only, and that is enough for us with that. And these are called secondary [prohibitions].
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with his daughter-in-law volitionally — and there are witnesses and a warning — is [punished] with stoning; and if there are no witnesses, with excision; inadvertently, he brings a fixed sin-offering. And if he laid with the daughter-in-law of his son or the son of his son, to the end of the world — or the daughter-in-law of his daughter — we strike him with lashes of rebellion.
עֶרְוַת כַּלָּתְךָ לֹא תְגַלֵּה – שֶׁלֹּא לָבוֹא עַל אֵשֶׁת הַבֵּן, שֶׁתִּהְיֶה אִשְׁתּוֹ מִן הָאֵרוּסִין, וְכָל שֶׁכֵּן מִן הַנִּשּׂוּאִין, בֵּין בְּחַיָּיו וַאֲפִילּוּ גֵּרְשָׁהּ, בֵּין אַחַר מוֹתוֹ, בְּכָל עִנְיָן זֶה נִקְרֵאת אֵשֶׁת בְּנוֹ, וַאֲסוּרָה עָלָיו, שֶׁנֶּאֱמַר (ויקרא יח טו) עֶרְוַת כַּלָּתְךָ לֹא תְגַלֵּה. וּמִן הַדּוֹמֶה שֶׁהַחִיּוּב אֲפִלּוּ בְּבֵן מַמְזֵר, בְּנוֹ הוּא מִכָּל מָקוֹם, אֲבָל בֶּן שִׁפְחָה וְנָכְרִית אֵין זֶה נִקְרָא בְּנוֹ לְשׁוּם דָּבָר.
The nakedness of your daughter-in-law you shall not reveal: To not have intercourse with the wife of the son, which is his wife from the betrothal — and all the more so from the marriage — whether during his lifetime and even after he divorced her, or whether after his death. In any manner [above], she is called the wife of his son and she is forbidden to him, as it is stated (Leviticus 18:15), “The nakedness of your daughter-in-law you shall not reveal.” And from that which appears, the liability is even with a son that is a mamzer — he is his son regardless. But the son from a maidservant or from a gentile is not called “his son” for anything.
שֹׁרֶשׁ אִסּוּר הָעֲרָיוֹת, כְּתַבְנוּהוּ (מצוה קצ).
We have written the root of the prohibition (Sefer HaChinukh 190).
מִדִּינֵי הַמִּצְוָה. כְּגוֹן מַה שֶׁאָסְרוּ זִכְרוֹנָם לִבְרָכָה (יבמות כא, ב) לְגֶדֶר זֶה כַּלַּת בְּנוֹ, וְכֵן כַּלַּת בֶּן בְּנוֹ עַד סוֹף הָעוֹלָם, וְאָסְרוּ גַּם כֵּן כַּלַּת בִּתּוֹ בִּלְבַד. וְהַטַּעַם כְּעֵין מַה שֶּׁאָמַרְנוּ לְמַעְלָה, לְפִי שֶׁהָאִסּוּר בָּא בַּכָּתוּב בְּכַלָּתוֹ גָּזְרוּ הֵם בְּכָל הַכַּלּוֹת הַבָּאוֹת מֵחֲמַת הַבֵּן, וְגָזְרוּ (מֵחֲמַת) [בְּכַלַּת] בִּתּוֹ בִּלְבַד, וְדַי לָנוּ בְּכָךְ, וְאֵלּוּ נִקְרָאוֹת שְׁנִיּוֹת.
From the laws of the commandment is, for example, that which they, may their memory be blessed, forbade (Yevamot 21b) — to make a fence for this — the daughter-in-law of his son and likewise the daughter-in-law of the son of his son, to the end of the world. And they also forbade only the daughter-in-law of his daughter. And the reason is similar to that which we said above — since the prohibition came in the verse about his daughter-in-law, they decreed about all daughters-in-law that come on account of the son; and they decreed with [the daughter-in-law of] the daughter only, and that is enough for us with that. And these are called secondary [prohibitions].
וְנוֹהֵג אִסּוּר זֶה בְּכָל מָקוֹם וּבְכָל זְמַן. וְהָעוֹבֵר עָלֶיהָ וְשָׁכַב אֶת כַּלָּתוֹ בְּמֵזִיד וְיֵשׁ עֵדִים וְהַתְרָאָה בִּסְקִילָה, וְאִם אֵין עֵדִים בְּכָרֵת, בְּשׁוֹגֵג מֵבִיא חַטָּאת קְבוּעָה. וְאִם שָׁכַב אֶת כַּלַּת בְּנוֹ אוֹ בֶּן בְּנוֹ עַד סוֹף הָעוֹלָם אוֹ כַּלַּת בִּתּוֹ, מַכִּין אוֹתוֹ מַכַּת מַרְדּוּת.
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with his daughter-in-law volitionally — and there are witnesses and a warning — is [punished] with stoning; and if there are no witnesses, with excision; inadvertently, he brings a fixed sin-offering. And if he laid with the daughter-in-law of his son or the son of his son, to the end of the world — or the daughter-in-law of his daughter — we strike him with lashes of rebellion.