ויקרא, פרק י״ח, פסוק י״ז

The Torah establishes a severe boundary protecting the sacred bond between closely related women, strictly forbidding a man from engaging in sexual relations with a woman and her mother, daughter, or granddaughter. This restriction is rooted not in the man's direct bloodline, but in the intimate biological and familial connection shared among the women themselves. To violate this boundary is to introduce deep moral corruption into the family unit. Because the women are an extension of one another's own flesh, it is considered a profound disgrace for a woman to share a bed with the same man who was intimate with her mother or daughter [רש״י, טור, תולדות יצחק, ביאור יש״ר]. Just as this restriction protects the generations below, such as a daughter or granddaughter, it equally applies to the generations above, including a mother-in-law or grandmother-in-law [תורה תמימה, רד״צ הופמן].

The legal framework of this restriction hinges on the establishment of a formal marital bond. Because the act is framed in the context of taking a wife, the restriction regarding a woman's relatives applies specifically when the man is lawfully married to her. If a relationship occurs outside of any marital framework, such as in a case of assault, her relatives do not become biblically forbidden to him [רש״י, מזרחי, רלב״ג, שפתי חכמים]. However, the moment he marries a woman, all of her direct female ancestors and descendants become permanently off-limits to him, whether through subsequent marriage or promiscuity [רד״צ הופמן]. This permanent barrier remains in full effect even after the wife passes away. Her descendants are considered her enduring remnant in the world, and thus the prohibition remains firmly in place [הכתב והקבלה].

The extreme severity of this act is categorized as a deep moral perversion. One perspective suggests that because the man shares no direct blood tie with his wife's relatives, his lower desires might tempt him into falsely believing they are permissible. Therefore, the Torah explicitly warns against this internal, sinful counsel [רש״י, תולדות יצחק]. The primary approach among commentators, however, links this perversion to hidden, corrupt thoughts. Due to the intense physical and familial resemblance between a woman and her mother or daughter, a man intimately involved with both might easily think of one while being with the other. This psychological confusion defiles the heart, completely corrupts the sanctity of marriage, and inflicts fatal damage on the natural bond between mother and daughter [רמב״ן, ספורנו, שד״ל, טור, ביאור יש״ר, שטיינזלץ].

Beyond the psychological damage, this distorted dynamic acts as a spiritual and moral barrier. Just as a muzzle restrains an animal, this behavior shuts down the natural, healthy purpose of human existence, dragging a person into moral degradation and an addiction to the lowest possible desires [הכתב והקבלה, רש״ר הירש]. Because this profound corruption stems from a single, unified violation of the family structure, all the forbidden women in this category are treated as one collective prohibition. Consequently, if a man were to tragically violate this boundary with all of them simultaneously, he is held accountable for a single, overarching transgression regarding his punishment [תורה תמימה].