Negative commandment · Lo Ta’aseh #342 of 613
Not to have intimate relations with the wife of one's father's brother
שלא לגלות ערוות אשת אחי האב
The verse · Vayikra 18:14
עֶרְוַת אֲחִי־אָבִיךָ לֹא תְגַלֵּה אֶל־אִשְׁתּוֹ לֹא תִקְרָב דֹּדָתְךָ הִוא
“Thou shalt not uncover the nakedness of thy father’s brother, thou shalt not approach to his wife: she is thy aunt.”
The Rambam’s words
Not to have intimate relations with the wife of one's father's brother, as [Leviticus 18:14] states: "Do not commit incest with his wife. [She is your aunt.]"
שלא לגלות ערוות אשת אחי האב, שנאמר "אל אשתו לא תקרב, דודתך היא" (ויקרא יח,יד).
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