ויקרא, פרק י״ח, פסוק י״ב

A family is built on layers of closeness and respect, and the boundaries of intimate relationships extend beyond the immediate nuclear household. Setting clear limits with slightly more distant blood relatives begins with the aunt, placing her first among this extended group [רד צ הופמן]. The restriction against intimate relations with a father's sister is absolute and comprehensive. It applies regardless of the exact nature of her sibling bond with the father. Whether she shares both parents with him, or is only a half-sister through his father or his mother, the ban remains fully in effect [תורה תמימה, ביאור יש״ר, רד צ הופמן]. Her legal status at birth does not change this boundary, as the rule applies whether she was born within a formal marriage or out of wedlock [תורה תמימה].

The underlying reason for this restriction is that the aunt is the close flesh of the father [אבן עזרא, ביאור שטיינזלץ]. The relationship is defined primarily through the father rather than the nephew because the connection between the nephew and his aunt is entirely rooted in his father. Since a sister is considered half of her brother's flesh, and a father and son are viewed as a single body, the aunt naturally becomes the close flesh of the son by extension [הכתב והקבלה, רד צ הופמן]. She is defined strictly as a first-degree blood relative rather than being equated directly with the father's own intimate vulnerability. That specific level of absolute union is reserved for the father's wife, who is viewed as becoming entirely one flesh with her husband [רד צ הופמן].

This framework highlights an interesting distinction regarding gender dynamics in family relationships. While a nephew is strictly forbidden from marrying his aunt, there is no parallel ban preventing an uncle from marrying his niece. The difference stems from natural social and family realities. A woman traditionally brings her nephew close to her, and he is frequently in her presence as part of his upbringing. This deep, ongoing closeness creates a higher risk of leading to inappropriate behavior, prompting a strict boundary to prevent it from the start. In contrast, a man typically does not share the same level of constant, close-knit involvement in raising his niece, making the concern significantly lower [בכור שור].