Such marriage is a clear violation of the Word of God.
Matthew Henry on Leviticus 18:16:
The relations forbidden are most of them plainly described; and it is generally laid down as a rule that what relations of a man’s own he is bound up from marrying the same relations of his wife he is likewise forbidden to marry, for they two are one. That law which forbids marrying a brother’s wife (v. 16) had an exception peculiar to the Jewish state, that, if a man died without issue, his brother or next of kin should marry the widow, and raise up seed to the deceased (Deu. 25:5), for reasons which held good only in that commonwealth; and therefore now that those reasons have ceased the exception ceases, and the law is in force, that a man must in no case marry his brother’s widow.
Gill on Leviticus 18:16:
Thou shalt not uncover the nakedness of thy brother's wife,.... Neither debauch her nor after the death of the brother marry her, that is, unless he dies without issue; and then, by another law, he was obliged to marry her,
Deuteronomy 25:5; hence the Targum of Jonathan adds; by way of explanation."in the life of thy brother, or after his death, if he has children,''but then that law was but an exception from this general rule, and so did not make it void in other respects, but bound it the more strongly; and besides, it was a special and peculiar law to the Jews, until the Messiah came to make it manifest of what tribe and family he came; and the reason of it ceasing, the law itself is ceased, and so neither binding on Jews nor Gentiles:
hence John the Baptist boldly told Herod to his face, that it was not lawful for him to have his brother's wife Matthew 14:3; and even such marriages were condemned by the very Heathens: Dionysius Halicarnassensis (n) relates, that Lucius Tarquinius, Superbus, his brother being removed by poison, took Tullia to wife, whom his brother Aruntus had before married; but the historian calls it , "an unholy marriage", and abominable both among Greeks and Barbarians: Plutarch also reports (o), that Marcus Crassus married the wife of his deceased brother; but such marriages are condemned by the same writer, as they are by the ancient Christians in their councils and canons (p); now by this same law, if it is not lawful for a man to have his brother's wife, then it is not lawful for her to have her sister's husband; or, in other words, if it is not lawful for a woman to marry two brothers, then it is not lawful for a man to marry two sisters: the case of Jacob will not countenance such a marriage, since he was imposed upon and deceived; and such marriages have also been disapproved of by the Heathens and Christians: Honorius the emperor married two daughters of Stilico, one after another, but the unhappy exit of both sisters showed that those marriages were not approved of by God, for they both died premature deaths, leaving no children
Notice the reference to Herod being rebuked for his marriage to his brother's wife.
Gill on Leviticus 20:21:
And if a man shall take his brother's wife,.... To his wife, whether in his life, as the Targum of Jonathan adds, or whether after his death, unless when there is no issue, then he was obliged to it by another law,
Deuteronomy 25:5; which is now ceased, and the law in
Leviticus 18:16; here referred to, stands clear of all exceptions:
it is an unclean thing; or a "separation" (k) from which a man should remove and keep at a distance, as from menstruous women, of whom this word is used; and so denotes that it is by all means to be avoided, as an abominable and detestable thing; and it is observed that of all copulations it is only used of this: and the Jewish writers, as Aben Ezra and others, observe that this case is somewhat like that of a menstruous woman, who in the time of her separation is unlawful, but when out of it lawful; and so, in this case, a brother's wife might not be taken, he being alive; but after his death she might, if she had no son, according to the law before referred to, but that is now abolished:
he hath uncovered his brother's nakedness; his wife's, which was his brother's; which through nearness of kin, he ought not to have done; and the same holds good of a wife's sister, the relation being the same:
they shall be childless; they shall have none by such a marriage or copulation, and die without any; and as this supposes the brother's wife to have children by her first husband, or otherwise while the Jewish law lasted, it would not have been unlawful to marry her husband's brother; the meaning may be, that these should die before them, or rather, as some think, those that might be born of such a marriage should not be reckoned legitimate, and so not inherit.
Clearly, then, levirate marriage was the temporary exception and the prohibition on marrying a deceased sibling's husband/wife is the rule which is still in force. It is clear from Leviticus 18:6-18 that the prohibition on the ground of affinity goes out equally far with that on the ground of consanguinity. This was the almost universal belief of the Christian church in all past ages.
