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.

Wesley's Notes

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.

King James Translators' Notes

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

Jamieson-Fausset-Brown Bible Commentary

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.

Matthew Henry's Concise Commentary

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.