To vex her - literally, to "bind" or "pack together". The Jewish commentators illustrate this by the example of Leah and Rachel Genesis 29:30.

A wife to her sister - Thou shalt not marry two sisters at the same time, as Jacob did Rachel and Leah; but there is nothing in this law that rendered it illegal to marry a sister-in-law when her sister was dead; therefore the text says, Thou shalt not take her in her life time, to vex her, alluding probably to the case of the jealousies and vexations which subsisted between Leah and Rachel, and by which the family peace was so often disturbed. Some think that the text may be so understood as also to forbid polygamy.

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 his wife, she having a sister living, but not to take one sister to another, or marry his first wife's sister, whether, as Maimonides (s) says, she was sister by father or mother's side, in marriage or in fornication:

to vex her, to uncover her nakedness; two reasons are given, why, though polygamy, or having more wives than one, was connived at, yet it was not allowed that a man should have two sisters; partly, because they would be more apt to quarrel, and be more jealous and impatient of one another, if more favour was shown or thought to be shown to one more than another; and partly, because it was a filthy and unbecoming action to uncover the nakedness of one, or lie with one so nearly related to his wife:

besides her in her life time; from whence some have concluded, and so many of the Jewish writers (t), that a man might marry his wife's sister after her death, but not while she was living; but the phrase, "in her lifetime", is not to be joined to the phrase "thou shall not take a wife"; but to the phrases more near, "to vex her in her lifetime", or as long as she lived, and "to uncover her nakedness by her" (u), on the side of her, as long as she lived; for that a wife's sister may be married to her husband, even after her death, cannot be lawful, as appears from the general prohibition, Leviticus 18:6; "none of you shall approach to him that is near of kin to him"; and yet it is certain that a wife's sister is near akin to a man; and from the prohibition of marriage with an uncle's wife, with the daughter of a son-in-law, or of a daughter-in-law, Leviticus 18:14; now a wife's sister is nearer of kin than either of these; and from the confusion that must follow in case of issue by both, not only of degrees but appellation of kindred; one and the same man, who as a father of children, and the husband of their mother's sister, stands in the relation both of a father and an uncle to his own children; the woman to the children of the deceased sister stands in the relation both of a stepmother, and of a mother's sister or aunt, and to the children that were born of her, she stands in the relation both of a mother and an uncle's wife; and the two sorts of children are both brethren and own cousins by the mother's side, but of this See Gill on Leviticus 18:16 for more; some understand this of a prohibition of polygamy, rendering the words, "thou shall not take one wife to another"; but the former sense is best; polygamy being not expressly forbidden by the law of Moses, but supposed in it, and winked at by it; and words of relation being always used in all these laws of marriage, in a proper and not in an improper sense: there is a pretty good deal of agreement between these laws of Moses and the Roman laws; by an edict of Dioclesian and Maximian (w), it was made unlawful to contract matrimony with a daughter, with a niece, with a niece's daughter, with a grandmother, with a great-grandmother, with an aunt by the father's side, with an aunt by the mother's side, with a sister's daughter, and a niece from her, with a daughter-in-law to a second husband, with a mother-in-law, with a wife or husband's mother, and with a son's wife; and several of these laws are recommended by Phocylydes, an Heathen poet, at least in a poem that hears his name; and the marriage of a wife's sister after her death has been condemned by several Christian councils (x).

(s) Hilchot Issure Biah, c. 2. sect. 9. (t) Misn. Yebamot, c. 4. sect. 13. Vajikra Rabba, sect. 22. fol. 164. 1. Peaicta, Ben Gersom in loc. (u) "apud vel prope eam"; so is sometimes used; see Nold. part. Concord. Ebr. p. 691. (w) Apud Mosaic. & Roman. Leg. Collat. ut supra. (Titus 6. a Pithaeo) (x) Concil. Illiber. can. 61. Aurat. can. 17. Auxer. can. 30.

Lastly, it was forbidden to take a wife to her sister (עליה upon her, as in Genesis 28:9; Genesis 31:50) in her life-time, that is to say, to marry two sisters at the same time, לצרר "to pack together, to uncover this nakedness," i.e., to pack both together into one marriage bond, and so place the sisters in carnal union through their common husband, and disturb the sisterly relation, as the marriage with two sisters that was forced upon Jacob had evidently done. No punishment is fixed for the marriage with two sisters; and, of course, after the death of the first wife a man was at liberty to marry her sister.

Neither shalt thou take a wife to her sister, to {i} vex her, to uncover her nakedness, beside the other in her life time.

(i) By seeing your affection more bent to her sister than to her.

18:18 Thou shalt not take a wife to her sister - Perhaps this text doth not simply forbid the taking one wife to another, but the doing it in such a manner or for such an end, that he may vex or punish, or revenge himself of the former; which probably was a common motive amongst that hardhearted people to do so.

a wife...: or, one wife to another

18. Neither shalt thou take a wife to her sister, to vex her-The original is rendered in the Margin, "neither shalt thou take one wife to another to vex her," and two different and opposite interpretations have been put upon this passage. The marginal construction involves an express prohibition of polygamy; and, indeed, there can be no doubt that the practice of having more wives than one is directly contrary to the divine will. It was prohibited by the original law of marriage, and no evidence of its lawfulness under the Levitical code can be discovered, although Moses-from "the hardness of their hearts" [Mt 19:8; Mr 10:5]-tolerated it in the people of a rude and early age. The second interpretation forms the ground upon which the "vexed question" has been raised in our times respecting the lawfulness of marriage with a deceased wife's sister. Whatever arguments may be used to prove the unlawfulness or inexpediency of such a matrimonial relation, the passage under consideration cannot, on a sound basis of criticism, be enlisted in the service; for the crimes with which it is here associated warrant the conclusion that it points not to marriage with a deceased wife's sister, but with a sister in the wife's lifetime, a practice common among the ancient Egyptians, Chaldeans, and others.

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.