Says the Gemara [3b]:
מנה"מ דת"ר (ויקרא יח, יח) אשה אל אחותה לא תקח לצרור לגלות ערותה עליה בחייה עליה מה ת"ל
From where are these matters, that if one’s forbidden relative comes before him for levirate marriage he is prohibited from marrying her or her rival wife, derived? It is as the Sages taught with regard to the verse: “And you shall not take a woman to her sister, to be a rival to her, to uncover her nakedness, with her in her lifetime” (Leviticus 18:18). What is the meaning when the verse states the apparently superfluous phrase: “With her”?
לפי שנאמר (דברים כה, ה) יבמה יבא עליה שומע אני אפי' באחת מכל עריות האמורות בתורה הכתוב מדבר נאמר כאן עליה ונאמר להלן עליה
The baraita explains: Since it is stated with regard to the wife of a deceased brother: “Her brother-in-law will have intercourse with her” (Deuteronomy 25:5), I would derive that when the verse speaks of the mitzva of levirate marriage, it includes even any one of those with whom relations are forbidden, as mentioned in the Torah. Therefore, one derives a verbal analogy: It is stated here, with regard to a wife’s sister: “With her,” and it is stated there, with regard to a levirate marriage: “With her.”
מה להלן במקום מצוה אף כאן במקום מצוה ואמר רחמנא לא תקח
The baraita explains the verbal analogy. Just as there, a levirate marriage involves the performance of a mitzva, so too, here, the statement “uncover her nakedness with her” includes the performance of a mitzva, and the Merciful One states in the Torah: “You shall not take.” The phrase “with her” teaches that even in a case where there is an obligation of levirate marriage, the Torah prohibition proscribing forbidden relatives remains in force.
ואין לי אלא היא צרתה מנין ת"ל לצרור ואין לי אלא צרתה צרת צרתה מניין ת"ל לצרור ולא לצור
The baraita continues: And I have derived only that she, his wife’s sister, is exempt from levirate marriage; from where do I derive that her rival wife is also exempt? The verse states: “To be a rival to her” (Leviticus 18:18), which indicates that not only is she prohibited, but so too is her rival wife. And I have derived only her rival wife; from where is it derived that the rival wife of her rival wife is also exempt? The verse states: “To be a rival [litzror],” using the full spelling with a double reish, and not latzor; this indicates that there are several rival wives, one after another.
Asked Rebbe Akiva Eiger: Once you have a pasuk to forbid her צרה, then the צרה has the status of אשת אח שלא במקום מצוה, so OF COURSE the צרת צרה will be forbidden, as she is the צרה of an ערוה!! Why then do we need a לימוד from the pasuk?
