leviticus 18:18 Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time. - Free Bible Online

      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.

JFB.