The prohibition of marrying a brother's wife presents a unique complexity within the laws of forbidden relationships. While it is strictly forbidden under most circumstances, it can become a mandated commandment under specific conditions. Ordinarily, a formal marriage cannot legally take effect within a forbidden relationship. However, because this specific union is permitted if the brother dies childless, the warning emphasizes that one must not marry her in any other situation, such as if the brother divorced her or passed away leaving children [ביאור יש״ר, ביאור שטיינזלץ].
To define the nature of this act, the relationship is categorized using a concept typically associated with a menstruating woman. Commentators offer varying perspectives on this comparison. The linguistic approach suggests the term does not refer to literal physical impurity, but stems from a root meaning distance and ostracism, indicating that the act is repulsive and must be avoided [רש״י, שד״ל, רבנו בחיי, אבן עזרא, ביאור שטיינזלץ, פירושי רד צ הופמן].
A more conceptual approach focuses on the parallel trajectory of the two situations. Just as a menstruating woman is temporarily forbidden but later permitted to her husband, a brother's wife is forbidden during his lifetime but may become permitted after his death if he dies childless. Because of this potential opening, a person might mistakenly assume it is acceptable to marry the widow even if the brother left children, perhaps out of a well-meaning desire to care for them and act as a father figure. The comparison clarifies that despite a potential window of permission, the penalty during the forbidden time remains severe [הטור הארוך, רבנו בחיי, מלבי״ם, בכור שור, חזקוני, אבן עזרא, הכתב והקבלה, רש ר הירש]. Furthermore, this parallel implies the law specifically addresses a paternal brother's wife, as only she is subject to the potential commandment to marry her brother-in-law, unlike a maternal brother's wife, whose exact status is subject to further Talmudic debate [תורה תמימה על התורה, מלבי״ם, העמק דבר, אדרת אליהו, פירושי רד צ הופמן].
From a legal standpoint, some sages initially use this same comparison to teach that the prohibition is fully violated at the very onset of the physical act [רש״י, הטור הארוך, רבנו בחיי, בכור שור]. However, later commentators point out that the final Talmudic conclusion derives this specific legal detail from a broader principle regarding all forbidden relationships, rather than from this specific instance [תורה תמימה על התורה, משכיל לדוד, הכתב והקבלה].
The prescribed penalty for this transgression is childlessness. Commentators draw a careful distinction between the phrasing used here and the penalty detailed in the preceding law regarding an aunt. If a person already has children when committing the offense, the prior penalty dictates that he will tragically outlive them and ultimately die childless. Conversely, if he commits this sin before having children, the phrasing establishes that he will simply remain in that state and never have children at all [רבנו בחיי, ביאור יש״ר, פירושי רד צ הופמן].
Taking a broader view of the chapter, these forbidden relationships were already introduced in an earlier section and are repeated here specifically to outline their respective punishments. Noticeably absent from this list, however, is the punishment for marrying a wife's sister. Some explain that this omission was intentional, out of respect for the patriarch Jacob, who married two sisters [רבנו בחיי, פירושי רד צ הופמן].