Leviticus 18:11

You must not have sexual intercourse with the daughter of your father’s wife born of your father; she is your sister. You must not have intercourse with her. ¹ 

¹ tn Heb “The nakedness of the daughter of your father’s wife born of your father, she is your sister; you must not uncover her nakedness.” That is, a half sister, the daughter of the man’s father by another wife, who is not the man’s mother, is to be considered a true sister. Therefore, the man must not have sexual intercourse with her.