Leviticus 20:17-18

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.

18If a man has sexual intercourse with a menstruating woman and uncovers her nakedness, he has laid bare her fountain of blood and she has exposed the fountain of her blood, so both of them must be cut off from the midst of their people.