Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 340
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 340, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אחות האב, שנאמר "ערוות אחות אביך, לא תגלה" (ויקרא יח,יב).
Not to have intimate relations with one's father's sister, as [Leviticus 18:12] states: "Do not commit incest with your father's sister."