You must not take your wife’s sister as a rival wife and have sexual relations with her while your wife is still alive.
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.
These laws relate to the seventh commandment, and, no doubt, are obligatory on us under the gospel, for they are consonant to the very light and law of nature: one of the articles, that of a man's having his father's wife, the apostle speaks of as a sin not so much as named among the Gentiles, Co1 5:1.
Commenting on Leviticus 18:6-18
Neither shalt thou take a wife to her sister,.... Both of them together, as Jarchi; two sisters at one and the same time; so the Targum of Jonathan,"a woman in the life of her sister thou shall not take;''that is, in marriage, that sister being his wife; for the sense of the Targumist can never be that a man might not take a woman for...