The nakedness of the sister of your mother you shall not reveal: To not have intercourse with the sister of the mother, as it is stated (Leviticus 18:13), “The nakedness of the sister of your mother you shall not reveal, etc.” All of its content is like the sister of the father (Sefer HaChinukh 197).
עֶרְוַת אֲחוֹת אִמְּךָ לֹא תְגַלֵּה – שֶׁלֹּא לָבוֹא עַל אֲחוֹת אֵם, שֶׁנֶּאֱמַר (ויקרא יח יג) עֶרְוַת אֲחוֹת אִמְּךָ לֹא תְגַלֵּה וְגוֹ'. כָּל עִנְיָנָהּ, כְּמוֹ אֲחוֹת אָב (מצוה קצז).
The nakedness of the sister of your mother you shall not reveal: To not have intercourse with the sister of the mother, as it is stated (Leviticus 18:13), “The nakedness of the sister of your mother you shall not reveal, etc.” All of its content is like the sister of the father (Sefer HaChinukh 197).