Lev Leviticus, chapter Book of Leviticus, chapter 20
If a man marries his sister, whether the daughter of his father or of his mother, and they have sexual relations,20:17 Literally and he sees her nakedness and she sees his nakedness 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.