Matthew Poole and continuators on Leviticus 18:16

16 Thou shalt not uncover the nakedness of thy brothers wife : it is thy brothers nakedness.

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

Marginal references: chap. 20. 21. Matth. 14. 4.