Westminster Confession of Faith 24.4

Chapter 24 — Of Marriage and Divorce

Section 4. Marriage ought not to be within the degrees of consanguinity or affinity forbidden in the Word; nor can such incestuous marriages ever be made lawful by any law of man, or consent of parties, so as those persons may live together as man and wife. The man may not marry any of his wife's kindred nearer in blood than he may of his own, nor the woman of her husband's kindred nearer in blood than of her own.