Leviticus 20:17-18

17‘If a man takes his sister, his father’s daughter or his mother’s daughter, and sees her nakedness and she sees his nakedness, it is a wicked thing. And they shall be cut off in the sight of their people. He has uncovered his sister’s nakedness. He shall bear his guilt.

18If a man lies with a woman during her sickness and uncovers her nakedness, he has exposed her flow, and she has uncovered the flow of her blood. Both of them shall be cut off from their people.