Charles Ellicott Commentary Leviticus 18:8

"The nakedness of thy father`s wife shalt thou not uncover: it is thy father`s nakedness." — Leviticus 18:8 (ASV)

The nakedness of thy father’s wife. —While the former prohibition refers to the son’s own mother, this law is directed against illicit relations with his stepmother. Here we have an instance where the phrase “to uncover the nakedness” denotes both illicit relations and incestuous marriage. Accordingly, the administrators of the law during the Second Temple period defined it as follows: a man’s father’s wife is forever prohibited, whether she is simply betrothed or married to his father, whether she is divorced or not, or whether she is a widow. All connection with her on the part of the father’s son is forbidden. If he lies with her while her husband is alive, he is doubly guilty: first, because she is a close relative, 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 his father's wives (2 Samuel 16:20–23; 1 Kings 2:17). These acts were not incestuous marriage but adultery, since their husbands were alive and the wives were not divorced from them. This also includes the sin practiced among some of the Christians in Corinth, which consisted in sons actually marrying their divorced stepmothers during the lifetime of their fathers, a sin the Apostle denounced with such severity (1 Corinthians 5:1–4). Among the ancient Arabs, marriages with stepmothers were common. To this day, among some tribes in Africa, when a father is unable due to advanced age to attend to his young wives, he voluntarily gives them over to his eldest son. The Quran, however, like the Mosaic law, proscribes these marriages (Quran, 4:27).