Leviticus 20:17, 19

“ ‘If a man marries his sister, the daughter of either his father or his mother, and they have sexual relations, it is a disgrace. They are to be publicly removed from their people. He has dishonored his sister and will be held responsible.

“ ‘Do not have sexual relations with the sister of either your mother or your father, for that would dishonor a close relative; both of you would be held responsible.