Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 341
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 341, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אחות האם, שנאמר "ערוות אחות אימך, לא תגלה" (ויקרא יח,יג).
Not to have intimate relations with one's mother's sister, as [Leviticus 18:13] states: "Do not commit incest with your mother's sister."