WCF 24.4.
Marriage ought not to be within the degrees of consanguinity or affinity forbidden by the Word. (Lev. 18, 1 Cor. 5:1, Amos 2:7) Nor can such incestuous marriage ever be made by any law of man or consent of parties, so as those persons may live together as man and wife. (Mark 6:18, Lev. 18:24–28)