Leviticus 18:9

18:9 You must not have sexual intercourse with your sister, whether she is your father’s daughter or your mother’s daughter, whether she is born in the same household or born outside it; you must not have sexual intercourse with either of them.

Leviticus 20:17

20:17 “‘If a man has sexual intercourse with his sister, whether the daughter of his father or his mother, so that he sees her nakedness and she sees his nakedness, it is a disgrace. They must be cut off in the sight of the children of their people. He has exposed his sister’s nakedness; he will bear his punishment for iniquity.