The laws governing forbidden relationships generally present absolute, eternal boundaries. However, the restriction against marrying a wife's sister breaks from this pattern, introducing a limitation that depends entirely on time and family dynamics. God intervenes in the delicate fabric of family relationships to prevent a situation where natural blood ties and sisterly love deteriorate into bitter jealousy and hatred.
The primary approach among commentators is that the terminology used for this prohibition is rooted in the standard term for a second wife, which translates conceptually to a rival. Sisters are inherently meant to love one another. Bringing them under a single roof as wives to the same man transforms them into bitter rivals, inevitably sparking competition, hostility, and relentless conflict [רמב״ן, ספורנו, בכור שור]. Beyond mere jealousy, this dynamic creates an inseparable, tightly bound connection, forcing the two women into an unnatural state where they are unwillingly fused together around one man [הכתב והקבלה, רש״ר הירש].
While the act of taking a woman generally refers to formal marriage and betrothal, commentators clarify that this prohibition extends to any intimate relationship, even outside the bounds of marriage [רבנו בחיי, ראד״צ הופמן]. Furthermore, the restriction against joining them together means that if a man attempts to formally betroth two sisters simultaneously, the betrothal is completely invalid for both of them [רש״י, מזרחי, שפתי חכמים, גור אריה].
The unique nature of this law lies in its timeline. Unlike other familial relationships, such as a woman and her daughter, which remain permanently forbidden, a wife's sister is only prohibited while the first wife is alive. This restriction remains in full force even if the man divorces his wife; as long as she lives, her sister is forbidden to him [רש״י, ביאור יש״ר, שטיינזלץ]. Only upon the wife's death does the prohibition dissolve, allowing a widower to marry his deceased wife's sister, as the threat of jealousy and hatred vanishes with her passing [שד״ל, ספורנו, תורה תמימה, רלב״ג, הדר זקנים].
Beyond the straightforward prohibition, these laws contain profound implications for the rules of Levirate marriage. If a man's brother dies childless, leaving a widow who ordinarily requires a Levirate marriage, this obligation is entirely nullified if the widow happens to be the surviving brother's own wife's sister. The commandment to perform a Levirate marriage does not override this prohibition [מלבי״ם, הכתב והקבלה]. Moreover, this exemption extends to any other wives the deceased brother may have had. The Torah views the deceased brother's wives as a single, cohesive household. Therefore, if one of the widows is forbidden to the surviving brother, the entire household is exempt from the Levirate marriage and its associated release ceremony, leaving the other wives free to marry elsewhere [תורה תמימה, רש״ר הירש, ראד״צ הופמן].
Throughout history, some have attempted to interpret the phrasing of this law metaphorically, suggesting it means one woman to another to establish a sweeping, universal ban on polygamy. Commentators firmly reject this misunderstanding. The specific condition that the prohibition lasts only during the first wife's lifetime proves that the law refers to literal sisters, especially since the Torah explicitly permits a man to marry multiple women in other contexts [נתינה לגר, תורה תמימה, אם למקרא].
Finally, this law is deeply connected to the historical precedent of Jacob, who married two sisters, Rachel and Leah, before the Torah was given, a historical event that presents a conceptual challenge [אבן עזרא, ראד״צ הופמן]. Some commentators suggest that the specific phrasing of the prohibition is a precise reflection of Jacob's experience. Jacob originally desired Rachel and considered her his primary wife. Consequently, the prohibition is formulated specifically against taking a desired woman in addition to her sister who had already been given to him first [חנוכת התורה, ברכת אשר על התורה].