Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 345
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 345, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אחות אישה, שנאמר "ואישה אל אחותה, לא תיקח" (ויקרא יח,יח).
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."