Leviticus 20:17-18

17If a man marries his sister, whether the daughter of his father or of his mother, and they have sexual relations,1 it is a disgrace. They must be cut off in the sight of their people. He has uncovered the nakedness of his sister; he shall bear his iniquity.

18If a man lies with a menstruating woman and has sexual relations with her,2 he has exposed the source of her flow, and she has uncovered the source of her blood. Both of them must be cut off from among their people.