Leviticus 18:18 - Clarke's commentary and critical notes on the Bible

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