Adam Clarke on Leviticus 18:18

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.