Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 332
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 332, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אחות, שנאמר "ערוות אחותך . . . לא תגלה" (ויקרא יח,ט).
Not to have intimate relations with one's sister, as [Leviticus 18:9] states: "Do not commit incest with your sister, your father's daughter."