Negative commandment · Lo Ta’aseh #344 of 613
Not to have intimate relations with one's brother's wife
שלא לגלות ערוות אשת אח
The verse · Vayikra 18:16
עֶרְוַת אֵשֶׁת־אָחִיךָ לֹא תְגַלֵּה עֶרְוַת אָחִיךָ הִוא
“Thou shalt not uncover the nakedness of thy brother’s wife: it is thy brother’s nakedness.”
The Rambam’s words
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."
שלא לגלות ערוות אשת אח, שנאמר "ערוות אשת אחיך, לא תגלה" (ויקרא יח,טז).
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה