Leviticus 18:11

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.

Leviticus 20:11

20:11 If a man has sexual intercourse with his father’s wife, he has exposed his father’s nakedness. Both of them must be put to death; their blood guilt is on themselves.