Negative commandment · Lo Ta’aseh #332 of 613
Not to have intimate relations with one's sister
שלא לגלות ערוות אחות
The verse · Vayikra 18:9
עֶרְוַת אֲחוֹתְךָ בַת־אָבִיךָ אוֹ בַת־אִמֶּךָ מוֹלֶדֶת בַּיִת אוֹ מוֹלֶדֶת חוּץ לֹא תְגַלֶּה עֶרְוָתָן
“The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, their nakedness thou shalt not uncover.”
The Rambam’s words
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."
שלא לגלות ערוות אחות, שנאמר "ערוות אחותך . . . לא תגלה" (ויקרא יח,ט).
Who & when
“And [it] is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה