Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 352
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 352, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אחי האב עצמו, שנאמר "ערוות אחי אביך, לא תגלה" (ויקרא יח,יד).
Not to commit sodomy with one's father's brother, as [Leviticus 18:14] states: "Do not commit a sexual offense with your father's brother."