ויקרא, פרק י״ח, פסוק י״ג

The laws of forbidden relationships establish firm boundaries to protect the integrity of the family unit. In the case of a maternal aunt, the prohibition is rooted deeply in the close blood bond she shares with the individual's mother. This restriction applies to any blood-related maternal aunt, regardless of whether she and the mother share the same father or only the same mother [ביאור יש״ר, ביאור שטיינזלץ].

The underlying reason for this boundary is the aunt's status as the mother's close flesh and blood. Interestingly, the reasoning provided for this rule is introduced with a specific expression of absolute certainty. This is a subtle yet significant departure from the preceding law regarding a paternal aunt, where the reason is stated simply as a fact without an introductory term of certainty.

This distinction highlights a profound reality regarding how family ties are established and verified. A relationship traced through a father ultimately relies on a legal and social presumption. In contrast, a maternal bond is an undeniable, biological reality. Because maternal lineage is an absolute and proven fact, the reasoning for the prohibition is framed with language that reflects total certainty, underscoring the undeniable biological connection between a mother and her sister [שפתי כהן, ברכת אשר על התורה].