18. Neither shalt thou take a wife to her sister. By this passage certain froward persons pretend that it is permitted, if a man has lost his wife, to marry her own sister, because the restriction is added, not to take the one in the lifetime of the other. From whence they infer, that it is not forbidden that she should succeed in the place of the deceased. But they ought to have considered the intention of the legislator from his own express words, for mention is made not only of incest and filthiness, but of the jealousy and quarrels, which arise from hence. If it had merely been said, “Thou shalt not uncover her turpitude,” there would have been some color to their pretext, that the husband being a widower, he would be free to marry his wife’s sister; but, when a different object for the law is expressly stated, i e. , lest she, who was legally married, should be troubled by quarrels and contentions, it is plain that the license for polygamy is restricted by this exception, in order that the Israelites should be contented with one evil, and, at least, should not expose two sisters to hostile contention with each other. The condition of the first wife was already painful enough, when she was compelled to put up with a rival and a concubine; but it was more intolerable to be constantly quarrelling with her near relative. The name of sister is not, therefore, restricted, I think, to actual sisters, but other relations are included in it, whose marriages would not otherwise have been incestuous. In a word, it is not incest which is condemned, so much as the cruelty of a husband, if he chose to contract a further marriage with the near kinswoman of his wife. Nor can we come to any other conclusion from the words of Moses; for if the turpitude of a brother is uncovered when his brother marries his widow, no less is the turpitude of a sister uncovered when her sister marries her husband after her decease. But hence we plainly see the diabolical arrogance of the Pope, who, by inventing new degrees of kindred, would be wiser than God; whilst he also betrays his cunning, because from this kind of sport he made himself a fat game-bag.
Since from long custom it is established that cousins-german should not marry, we must beware of giving scandal lest too unbridled a liberty should expose the Gospel to much reproach; and we must bear in mind Paul’s admonition, to abstain even from things lawful when they are not expedient. (1 Corinthians 10:23.)