Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 342
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 342, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אשת אחי האב, שנאמר "אל אשתו לא תקרב, דודתך היא" (ויקרא יח,יד).
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.]"