Negative commandment · Lo Ta’aseh #345 of 613
Not to have intimate relations with one's wife's sister
שלא לגלות ערוות אחות אישה
The verse · Vayikra 18:18
וְאִשָּׁה אֶל־אֲחֹתָהּ לֹא תִקָּח לִצְרֹר לְגַלּוֹת עֶרְוָתָהּ עָלֶיהָ בְּחַיֶּיהָ
“Neither shalt thou take a wife to her sister, as her rival, and uncover her nakedness, beside the other in her life time.”
The Rambam’s words
Not to have intimate relations with one's wife's sister, as [Leviticus 18:18] states: "Do not take a woman as a wife together with her sister."
שלא לגלות ערוות אחות אישה, שנאמר "ואישה אל אחותה, לא תיקח" (ויקרא יח,יח).
Who & when
“And this warning (negative commandment) is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה