ויקרא, פרק י״ח, פסוק ט״ז

Family loyalty and boundaries often intersect in profound ways, especially regarding marriage within the immediate family. The rule against marrying a brother's wife establishes a firm boundary, yet it also contains a highly specific exception. The primary approach among commentators is that this restriction applies equally whether the brothers share a father or a mother. Even though a sister-in-law does not share a blood tie, she is strictly forbidden because her relationship is considered a direct extension of the brother himself [ביאור יש״ר].

There is, however, a unique exception to this strict boundary. If a brother dies without leaving any sons, the Commandment of levirate marriage comes into play, allowing the surviving brother to marry the widow [אבן עזרא, רד צ הופמן]. This exception is highly conditional. If the deceased brother had sons, or if the widow falls into another category of forbidden relatives, the original prohibition remains fully active and the marriage cannot take place [בכור שור].

The apparent contradiction between a severe restriction and a sudden permission is resolved by understanding the divine origin of these laws. God issued both of these seemingly opposite instructions in a single, unified utterance. At the exact moment the brother's wife was forbidden, He also created the exception for levirate marriage [תורה תמימה]. Historically, certain groups, such as the Samaritans and Karaites, rejected this exception entirely and maintained the absolute prohibition [רד צ הופמן].

The permanence of the restriction is emphasized by noting that the woman inherently remains in her forbidden state unless the specific conditions are met. While the wife of a paternal brother might become permitted through levirate marriage, the wife of a maternal brother remains strictly forbidden forever, even after her husband's death. Because the Commandment of levirate marriage only applies to paternal brothers, the maternal sister-in-law can never become permitted. Ultimately, without His explicit instruction to allow the exception, every sister-in-law would be permanently forbidden, just like any other prohibited relative [תורה תמימה].