Do Not Take Your Wife’s Sister as a Rival Wife

“Litsror” — To Make into a Rival

Leviticus 18:18: “Do not take your wife’s sister as a rival wife and have sexual relations with her while your wife is alive.” The key word is “litsror” — from the root tsarar, meaning to bind tightly or to make an adversary. The related word “tsarah” means a rival wife. The prohibition is not simply against marrying the wife’s sister; it is against creating the specific dynamic of competing sisterhood within a marriage. The Torah identifies the harm: it produces adversarial rivalry between sisters bound to the same man.

The verse specifies “in her lifetime” (be’cha-ye-ha) — the prohibition is conditional on the first wife being alive. This is the sharpest structural difference between commandment #520 and commandment #519: the wife’s daughter prohibition is permanent; the wife’s sister prohibition ends at the wife’s death. After the wife dies, her sister becomes an eligible marriage partner. The Talmud (Yevamot 8b) confirms: once the first wife dies, the prohibition dissolves entirely.

Jacob, Leah, and Rachel — The Pre-Sinai Precedent

Genesis 29:15–30: Laban deceived Jacob, substituting Leah for Rachel on the wedding night. Jacob woke to find he had married the wrong sister. He worked another seven years and received Rachel as well. Jacob was married simultaneously to two sisters — exactly the situation commandment #520 prohibits. The rivalry the Torah encodes in the word “litsror” is not theoretical: Genesis 29:30–31 immediately notes that Jacob loved Rachel more than Leah, and God saw Leah was unloved.

Genesis 30:1–24 narrates the competition between Leah and Rachel for children — each giving Jacob their handmaids, counting births as competitive victories. The rivalry the verse prohibits (litsror) is on full display. The pre-Sinai permission that produced Jacob’s family structure is exactly what Sinai closed. The Torah records the story precisely because the rivalry between Leah and Rachel — the suffering it produced — is the etiology of the prohibition.

The rabbis (Yevamot 62a) note that once Rachel died, Jacob's other unions were no longer subject to the sister-rivalry prohibition. The conditional structure of the commandment (“while your wife is alive”) was mirrored in the pre-Sinaitic story: the problem is the simultaneous living rivalry, not the subsequent union after one sister’s death.

The Living Wife Clause — How This Commandment Differs

Most prohibitions in Leviticus 18 are permanent. The wife’s sister prohibition is the only one with an explicit temporal clause. The rabbis (Yevamot 8b) derived from this that the prohibition is about the living relational conflict, not about the blood connection itself. The wife’s sister is not a blood relative — she is only prohibited because her simultaneous position as the man’s wife would create the specific harm of sisterly rivalry.

Karaite interpreters read the verse differently: they argued the prohibition is permanent, not ending at the wife’s death. But the rabbinic reading — confirmed by Yevamot throughout — holds that “be’cha-ye-ha” (in her lifetime) limits the prohibition. The death of the first wife dissolves the prohibition on her sister. This is the only commandment in Leviticus 18 where the marriage itself, rather than the kinship, creates the forbidden relationship — and therefore only while the marriage exists.