Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 344
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 344, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אשת אח, שנאמר "ערוות אשת אחיך, לא תגלה" (ויקרא יח,טז).
Not to have intimate relations with one's brother's wife, as [Leviticus 18:16] states: "Do not commit incest with your brother's wife."