The Gemara answers that these enumerations come to exclude those additions of Rav and of Rav Asi. Rav added the rival wife of a woman suspected by her husband of adultery [sota], while Rav Asi added the rival wife of an aylonit. The Gemara asks: And according to the opinion of Rav and according to the opinion of Rav Asi, the enumeration of the mishna comes to exclude what?

The Gemara responds: If each maintains in accordance with the opinion of the other, then one enumeration of the mishna comes to exclude the rival wife of a wife who performed refusal. If the deceased brother had two wives, one of whom was a minor, and she refused the yavam, her rival wife is prohibited from levirate marriage with him. However, the latter is not entirely exempt and must perform ḥalitza. And the other one comes to exclude the rival wife of the wife of one who remarries his divorcée, i.e., a woman who was illicitly remarried by her former husband after she had been married to another man.

And if Rav and Rav Asi do not each maintain in accordance with the opinion of the other, then one enumeration comes to exclude the opinion of the other, as they do not agree that the halakha stated by the other should be included in the mishna, and the other one comes to exclude one of the above suggestions, either the rival wife of a wife who performed refusal or the rival wife of the wife of one who remarries his divorcée.

The Gemara asks: If so, according to the opinion of Rav and according to the opinion of Rav Asi, let the tanna teach these cases. Since in their opinions there are more than fifteen women to whom the principle of the mishna applies, why weren’t they all stated by the tanna of the mishna? The Gemara answers: They were not taught because they do not completely fit all of the halakhic rulings here.

The Gemara elaborates: This is because they do not involve the case of a rival wife of a rival wife. With regard to the fifteen women listed, the discussion of the mishna concerning rival wives and rival wives of rival wives is appropriate. However, the two cases cited by Rav and Rav Asi do not leave room for such deliberations, as both a sota and an aylonit are exempt and forbidden equally to all of the brothers, because their prohibition does not result from a familial relation to one of the living brothers but from a personal issue relating to the women themselves. Since none of the brothers may marry her rival wife, there is no possibility of a rival wife of a rival wife, and consequently these cases were omitted from the mishna’s list of fifteen women.

§ After analyzing the order and language of the mishna, the Gemara discusses the halakhot themselves. 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”?

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.”

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.

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.

And I have derived from this verbal analogy only that the mitzva of levirate marriage does not apply to a wife’s sister. From where is it derived that the same applies to the other women with whom relations are forbidden? You can say as follows: Just as the case of a wife’s sister is specific in that she is a forbidden relative, and this is a prohibition for whose intentional violation, i.e., for intentional sexual relations with her, one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and she is prohibited to the yavam in levirate marriage; so too, with regard to all women with whom relations are forbidden by a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, they are prohibited to the yavam in levirate marriage.

The baraita continues: And I have derived only the cases of those women themselves with whom relations are forbidden; from where is it derived that their rival wives are also exempt from levirate marriage? You can say as follows: Just as a wife’s sister is specific in that she is a forbidden relative and this is a prohibition for whose intentional violation one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and she is prohibited to the yavam in levirate marriage, and her rival wife is likewise prohibited; so too, any woman with whom relations are forbidden and this is a prohibition for whose intentional violation one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and this woman is one who is forbidden to the yavam, her rival wife is likewise forbidden. The baraita concludes: From here the Sages stated that fifteen women exempt their rival wives and the rival wives of their rival wives from ḥalitza and from levirate marriage forever.

§ One might have thought that I should include in this principle even the six women with whom relations are forbidden that are more severe than these, i.e., one’s mother, his father’s sister, and so on, as stated in a later mishna (13b), since they too are forbidden by a prohibition entailing karet. This would mean that their rival wives should likewise be prohibited to enter into levirate marriage with this yavam as forbidden rival wives.

In response, you can say: Just as a wife’s sister is specific in that she is a forbidden relative and this is a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, and yet she is permitted to marry one of the brothers but she is prohibited to the yavam in levirate marriage, and her rival wife is likewise prohibited in levirate marriage; so too, with regard to any woman with whom relations are forbidden and this is a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, and who is permitted to marry one of the brothers but she is prohibited to the yavam in levirate marriage, her rival wife is also prohibited in levirate marriage.

This excludes the six women with whom relations are forbidden by a more severe prohibition than those, since they may not marry the brothers, i.e., they are forbidden to all of the brothers. One’s mother may never marry his brother, either because she is also that brother’s mother, or because she is his father’s wife. Consequently, their rival wives are permitted, as the halakha with regard to a rival wife applies only due to the brother. In other words, the prohibition against marriage to the rival wife of a forbidden relative is applicable only in instances of levirate bonds. When the levirate bond does not take effect at all, the rival wife is not forbidden.

The baraita adds: We have learned the warning concerning this prohibition that the yavam may not marry his forbidden relative from 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). From where is the punishment that he incurs if he transgresses and marries her derived? The verse states: “For whoever shall do any of these abominations, the souls that do them shall be cut off from among their people” (Leviticus 18:29).

§ Up until this point, the Gemara has cited the baraita that interprets the biblical basis for the halakha of the mishna. The Gemara further analyzes the foundational principles of this topic. The reason that these women are exempt from levirate marriage is that the Merciful One writes “with her,” from which it may be inferred that if that was not so, I would say that a wife’s sister enters into levirate marriage with her sister’s husband. What is the reason that one would have assumed that this is the case? It is as we say, in accordance with a principle, that a positive mitzva comes and overrides a prohibition. In this case, the positive mitzva to enter into levirate marriage overrides the prohibition against marrying one’s wife’s sister.

The Gemara asks if that principle is applicable in this case. One can say that we said that a positive mitzva comes and overrides a prohibition only when there is a prohibition for which one is punished by lashes alone. However, with regard to a prohibition that includes the punishment of karet, does a positive mitzva override it? This prohibition is more severe than a regular one, and therefore perhaps a positive mitzva does not override it. And furthermore, with regard to a prohibition for which one is punished by lashes alone, from where do we derive that a positive mitzva overrides it?