2,030. The Four Perpetually-Prohibited Relationships

Hilchos Issurei Biah 1:23

If a man says that his daughter is betrothed to a certain man, his word is accepted and she must marry that person. If she cheats on that man while betrothed, she isn’t stoned because of her father’s statement unless there are also witnesses that she was betrothed in their presence. Similarly, if a woman claims to have been betrothed, she isn’t executed for infidelity based on her own statement. Rather, there must be witnesses or an established presumption.

Hilchos Issurei Biah 2:1

Four women are considered perpetually-prohibited relationships to a man: his father’s wife, his son’s wife, his brother’s wife and his father’s brother wife. This applies after betrothal and after marriage, in the lifetime of their husbands and after their deaths, and even if the couple should divorce. The one exception is the wife of a brother who dies childless. If a man is intimate with one of these women during her husband’s lifetime, he is liable for two offerings: one for an incestuous relationship and one on account of a married woman. One is liable for both of these things because the prohibitions take effect simultaneously.