The eighth ease of incest is intercourse with a brother's wife. Yet this is commanded under certain circumstances in the Book of Deuteronomy, and was practiced in patriarchal times (Genesis 38:8). The following are the circumstances under which it is commanded.
"If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her" (Deuteronomy 25:5).
It has been asked, "How can the same thing be forbidden as immoral in Leviticus, and commanded as a duly in Deuteronomy?" Bishop Wordsworth replies, "In a special case, for a special reason applicable only to the Jews, God was pleased to dispense with that law, and in the plenitude of his omnipotence to change the prohibition into a command.
… God cannot command anything that is sinful. For sin is 'transgression of the Law' (1 John 3:4), and whatever he commands is right. But it would be presumptuous to say that we may dispense with God's law concerning marriage, because he in one case dispensed with it; as it would be impious to affirm that murder is not immoral, and may be committed by us, bemuse God, who is the sole Arbiter of life and death, commanded Abraham to slay his son Isaac."
The levirate marriage was not a concession to the desires of the second brother, but a duty enjoined for a family or tribal purpose, and it was plainly at all times must distasteful. Thus Onan refused to perform his duty to Er's wife (Genesis 38:9); the legislation in Deuteronomy anticipates objection on the part of the brother, and institutes an in-suiting ceremony to be gone through by him if he declines to do his duty to his dead brother (Deuteronomy 25:9, Deuteronomy 25:10), which we see carried out in some of its details in the case of Ruth's kinsman (Ruth 4:7, Ruth 4:10).
Indeed, in such a marriage, the second husband seems rather to have been regarded as the continuation of the first husband than as having a substantive existence of his own as a married man. He performed a function in order "that the name of his brother which is dead may not be put out of Israel" (Deuteronomy 25:6), "to raise up the name of the dead upon his inheritance, that the name of the dead be not cut off from among his brethren" (Ruth 3:10).
The second husband's position may be compared to that of the concubine presented by Rachel to her husband. "Behold my maid Bilhah, go in unto her; and she shall bear upon my knees, that I may also have children by her" (Genesis 30:3).
The whole object of the rule was that, as the elder brother could not keep up the flintily by begetting an heir, the younger brother should do it for him after his death.