Leviticus 18:18

In her life time] This implies that after the death of the first wife a man might marry her sister. It is not a law against polygamy but only against a special form of it, viz. marrying two sisters. The restriction is professedly made in the interests of domestic peace and happiness. For to vex her RV reads, 'to be a rival to her': cp. the case of Leah and Rachel, the wives of Jacob, who were sisters (Genesis 29:30).