It says, chap. xxiv. 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."
The answer to this query then must be: Yes, the Confession forbids the marriage. The relation is not expressly named, but it is implicitly stated. The argument is simply this: A man may not marry his own sister, therefore he may not marry his wife's sister.
2. On what ground is this prohibition based? It is not express prohibition — but INFERENCE. The inference is: As a man may not marry any of his wife's kindred nearer in blood than he may of his own; and as he may not marry his own sister, so he may not marry his wife's sister. The ground, then, on which the inference rests is the statement, "A man may not marry any of his wife's kindred nearer in blood than he may of his own."
3. We next enquire: Is this ground found in the Word of God; and particularly in the passage quoted in the Confession of Faith as proof?
Let us first examine the passage quoted by the Westminster divines as proof. We find under letter K, in small type, under the text of the Confession, the passage rested on as proof of the statement above given in italics. It reads in full:
"Lev. XX. 19-21. And thou shalt not uncover the nakedness of thy mother's sister, nor of thy father's sister; for he uncovereth his near kin; they shall bear their iniquity. And if a man shall lie with his uncle's wife, he hath uncovered his uncle's nakedness; they shall bear their sin; they shall die childless. And if a man shall take his brother's wife, it is an unclean thing, he hath uncovered his brother's nakedness; they shall be childless."
