Leviticus 20:17-18

17And if a man take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness, that is a disgrace; and they shall be cut off before the eyes of the children of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity.

18And if a man shall lie with a woman in her infirmity, and uncover her nakedness, her flux doth he lay bare, and she hath uncovered the fountain of her blood; and both of them shall be cut off from among their people.