Leviticus 18:18 - Joseph Benson’s Commentary on the Old and New Testaments

A wife to her sister The meaning seems to be, that no man should take to wife two sisters, which had sometimes been done, as we see in the example of Jacob. It may, however, signify that a man, who already had a wife, was not to take another out of mere incontinency, which would tend only to break his wife's peace; but that if he took that liberty at all, it ought only to be when his wife consented to it, as Sarah did in the case of Abraham's marrying Hagar, and Rachel in the case of Bilhah. To vex her Grotius justly observes, that as the feuds and animosities of brothers are, of all others, the most keen; so are generally the jealousies and emulations between sisters, whereof we have an example in the history of Rachel and Leah.