Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 333

שלא לגלות ערוות אחות מן האב ומן האם, שנאמר "ערוות בת אשת אביך מולדת אביך, אחותך היא--לא תגלה, ערוותה" (ויקרא יח,יא).

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."