The boundaries of intimacy within an extended family are delicate, carrying both deep trust and significant risk. A severe curse is directed at a person who violates these boundaries by engaging in sexual relations with a close female relative by marriage. There is a discussion regarding the exact identity of this relative. While the accepted understanding is that it refers to a mother-in-law, another perspective suggests it actually refers to a daughter-in-law [מלבי״ם].
Regardless of the specific relative, this act is considered one of the most severe sexual offenses, yet it occurs because of the unique dynamics of family life. The home environment breeds a level of closeness that is not always strong enough to prevent the sin, but provides enough intimacy to allow it to happen. Because of the intense shared shame, these acts are typically hidden and covered up rather than becoming public knowledge [מלבי״ם, ביאור שטיינזלץ]. The prohibition remains fully in effect whether the woman is married or unmarried [ביאור שטיינזלץ].
Beyond the sheer gravity of the sin, it brings immense disgrace [ביאור שטיינזלץ] and fatally damages the fabric of the family. A man who commits this act effectively turns his mother-in-law into a rival against her own daughter, his wife [שפתי כהן]. Approaching the relative as a mother-in-law, the warning also carries a broader message about family influence. It cautions a man against being overly swayed by his wife's advice, recognizing the natural reality that a daughter is deeply connected to her mother, which could improperly guide his heart [מלבי״ם]. On the other hand, viewing the relative as a daughter-in-law brings to mind the historical example of Judah, who exercised immense caution and completely avoided intimacy with Tamar once he realized her true identity as his daughter-in-law [מלבי״ם].
The severity of the curse remains even when death alters the family structure. The act is fully condemned even if it occurs after the father-in-law has passed away [העמק דבר]. There is also a legal discussion regarding the status of the sin if the man's wife has died. Even according to the opinion that a human court no longer administers the death penalty for this act after the wife's passing, the offender is still struck by this curse [העמק דבר]. Regarding the heavenly consequence in this scenario, some maintain that the punishment is limited strictly to the curse itself, while stricter opinions assert that the act still carries the severe heavenly punishment of being spiritually cut off [תיבת גמא].