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, De 25:5; which is now ceased, and the law in Le 18:16; here referred to, stands clear of all exceptions:it [is] an unclean thing; or a "separation" 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.