Lev Leviticus3. Mósebók, chapterkafli Book of LeviticusÞriðja Mósebók, chapterkafli 20

A man who marries his sister, whether she's his father's daughter or his mother's daughter, and they have sex, has done something shameful. They must be expelled from their people in front of everybody. He has brought shame on his sister; he bears the responsibility for his punishment.