Leviticus 18:8

The nakedness of thy father’s wife. — Whilst the former prohibition refers to the son’s own mother, this law is directed against illicit commerce with his stepmother. Here we have an instance where the phrase “to uncover the nakedness” denotes both illicit commerce and incestuous marriage. Accordingly the administrators of the law during the second Temple defined it as follows; a man’s father’s wife is for ever prohibited, whether she be simply betrothed or married to his father, whether she be divorced or not, whether she be a widow or not; all connection with her on the part of the father’s son is forbidden. If he lie with her while her husband is alive, he is doubly guilty, first, because she is near of kin, and secondly, because she is another man’s wife. This, therefore, includes the sin of Reuben with Bilhah, his father’s concubine (Genesis 35:22), and of Absalom with the wives of his father (2 Samuel 16:20; 1 Kings 2:17), which was not incestuous marriage but adultery, since their husbands were alive and the wives were not divorced from them, as well as the sin practised among some of the Christians in Corinth, which consisted in sons actually marrying their divorced stepmothers in the lifetime of their fathers, and which the Apostle denounced with such severity (1 Corinthians 5:1). Among the ancient Arabs, marriages with stepmothers were common, and to this day among some tribes in Africa, when a father is unable through advanced age to attend to his young wives, he voluntarily gives them over to his eldest son. The Koran, however, like the Mosaic law, proscribes these marriages (Koran, 4:27).