Leviticus 20:17-18

17Whoever takes his sister, his father’s daughter or his mother’s daughter, and has sexual intercourse does a shameful thing. They both must be publicly excluded from the people. He has had sexual intercourse with his sister and must be punished.

18If a man has sexual intercourse with a woman while she has her monthly period, both of them have had sexual intercourse in blood. They must be excluded from the people.