Leviticus 18:16

Neither in his lifetime, nor after his death, and therefore a woman might not marry her husband's brother, nor might a man marry his wife's sister, either before or after his wife's death, for so all the prohibitions are to be understood; which will give light to Leviticus 18:18. But God, who can undoubtedly dispense with his own laws, did afterwards make one exception to this rule, of which see Deuteronomy 25:5.