Negative commandment · Lo Ta’aseh #333 of 613
To not uncover the nakedness of his sister from the father, and she is the daughter of the wife of the father
שלא לגלות ערוות אחות מן האב ומן האם
The verse · Vayikra 18:11
עֶרְוַת בַּת־אֵשֶׁת אָבִיךָ מוֹלֶדֶת אָבִיךָ אֲחוֹתְךָ הִוא לֹא תְגַלֶּה עֶרְוָתָהּ
“The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness.”
The Rambam’s words
Not to have intimate relations with a sister, notes that this prohibition is almost identical with Negative Mitzvah 331, and explains that the Torah forbids such relations with two separate commands so that a person who commits such a sin will be held liable on two counts. [the daughter of] either your father or your mother, as [Leviticus 18:11] states: "She is the daughter of your father, your father's progeny, do not commit incest with her."
שלא לגלות ערוות אחות מן האב ומן האם, שנאמר "ערוות בת אשת אביך מולדת אביך, אחותך היא--לא תגלה, ערוותה" (ויקרא יח,יא).
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה