This works out well when the deceased brother had first married one of the sisters, who was thereby rendered prohibited to the yavam as his brother’s wife, and only afterward had the living brother married her sister, which renders his brother’s wife forbidden to him on another count, this time as his wife’s sister. As in that case one can say that since the first prohibition with regard to a brother’s wife was subsequently nullified and thereby permitted by the mitzva of levirate marriage, the prohibition with regard to a wife’s sister was also nullified and thereby permitted.

However, if the reverse occurred, and first the living brother had married his wife and only afterward the deceased brother had married her sister, in this case the prohibition with regard to a wife’s sister precedes the prohibition proscribing a brother’s wife, i.e., she was initially forbidden to him as his wife’s sister. Consequently, even when the second prohibition lapses upon the death of his childless brother, the first prohibition should remain intact.

And furthermore, even in a case where the deceased brother had married first, it works out well only when the deceased brother had married the woman and then died, and only afterward the surviving brother married that brother’s wife’s sister. The reason is that in this case, the sister of the wife of the deceased brother was fit for him in between, i.e., during the period from when the deceased brother died until he married that brother’s wife’s sister. Therefore, it can be claimed that the second prohibition should not apply. However, if the deceased brother married first, and did not die yet, and afterward, during the deceased brother’s lifetime, the living brother married the deceased brother’s sister, she was not fit for him at all, as there was no period of time during which she was permitted to him.

The Gemara provides proof for this claim: Doesn’t Ulla agree that if a leper experienced a seminal emission on the night of the eighth, not on the eighth day but on the previous night, that he may not insert his hands and feet to the thumb and big toe to receive the blood and oil for his purification? The reason is that in this case he is ritually impure due to the emission and he had not yet emerged from the impurity of leprosy at a time that is fitting to bring an offering, as he cannot sacrifice his offerings until the daylight hours of the eighth day. Since there was not a single moment when he was permitted to enter the Temple in his state of ritual impurity, in this case one cannot apply the principle: Since it is permitted, it is permitted.

§ Rather, the Gemara suggests that the phrase ''with her,'' which teaches the prohibition with regard to a wife’s sister in levirate marriage, is necessary for a case where the deceased brother had first married and then died, and only afterward the living brother married the sister of the deceased brother’s wife, in which case his brother’s wife was permitted to him for a period of time. Accordingly, one might have applied the principle: Since it is permitted, it is permitted. For this reason it was necessary to write ''with her,'' to teach that this second woman is nevertheless forbidden as his wife’s sister.

And if you wish, say instead that the suggestion that other women with whom relations are forbidden are permitted in levirate marriage is derived from the juxtaposition of Rabbi Yona. As Rabbi Yona said, and some say this was stated by Rav Huna, son of Rav Yehoshua: 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). In this verse all of the women with whom relations are forbidden are juxtaposed with the case of a brother’s wife. Consequently, one might say that just as a brother’s wife is permitted in levirate marriage, so too, all women with whom relations are forbidden are likewise permitted. Therefore, the Merciful One writes: ''With her,'' which excludes the prohibition with regard to a wife’s sister.

Against this proof, Rav Aḥa of Difti said to Ravina: Since the halakha of all women with whom relations are forbidden can be juxtaposed with the case of a brother’s wife and thereby rendered permitted in levirate marriage, and these cases can equally be juxtaposed with the halakha of a wife’s sister, which would mean that they are forbidden, what did you see that you juxtaposed them with a wife’s sister? Juxtapose them instead with a brother’s wife. If so, the opposite could be derived from the phrase ''with her,'' i.e., not that all women with whom relations are forbidden are prohibited like a wife’s sister, but that all these women are permitted in levirate marriage, even a wife’s sister.

The Gemara answers: If you wish, say that if one must decide between two possible ways of understanding juxtaposed cases, one of which leads to a lenient ruling and the other to a stringent ruling, we juxtapose to reach the stringent ruling, not the leniency. If you wish, say instead: Here, there are two prohibitions, a brother’s wife and a wife’s sister, and there, in the case of all other women with whom relations are forbidden, there are also two prohibitions. And one may derive a case involving two prohibitions from another case that involves two prohibitions. However, here, in the case of a regular levirate marriage that does not involve any other forbidden relation, there is only one prohibition proscribing a brother’s wife, and one may not derive a case involving two prohibitions from a case that involves only one.

§ Rava said that this entire halakha must be understood differently. With regard to a forbidden relation herself, it is not necessary for a verse to teach that she cannot enter into levirate marriage, as a positive mitzva does not override a prohibition that includes karet. Rather, the verse ''with her'' is necessary to prohibit a rival wife, as a rival wife is not prohibited to the yavam as a forbidden relation.

The Gemara asks: And does the tanna in fact maintain with regard to a forbidden relation that it is not necessary for a verse to teach that she is prohibited in levirate marriage? But isn’t it taught in that same baraita: I have derived nothing other than that this woman is exempt from levirate marriage. From where is it inferred that the same applies to her rival wife? This shows that the case of a forbidden relation does require a special inference from a verse.

The Gemara answers: It is due to her rival wife, i.e., the baraita does not mean that the halakha of women with whom relations are forbidden is derived from this verse; rather, this case is mentioned only to introduce the case of a rival wife.

The Gemara raises another difficulty: But it is taught in the baraita: I have derived nothing other than that these sisters are exempt from levirate marriage, which again suggests that the derivation from the verse applies to all of them. This is similarly rejected: This is also stated due to their rival wives.

The Gemara suggests: Come and hear a different baraita that contradicts Rava’s statement: Rabbi Yehuda HaNasi says that there is a different proof that a forbidden relation and her rival wife are prohibited in levirate marriage. The Torah says: ''He will have intercourse with her and take her to him to be his wife and consummate the levirate marriage with her'' (Deuteronomy 25:5). Since the verse does not say simply: Have intercourse, but: ''Have intercourse with her,'' this indicates that he takes this woman specifically, not a different woman who is forbidden. Furthermore, the verse does not simply state: And consummate the levirate marriage, but: ''And consummate the levirate marriage with her.'' These additions serve to prohibit rival wives and women with whom relations are forbidden.

This shows that even the prohibition with regard to women with whom relations are forbidden requires a special inference. The Gemara answers: Amend the language of the baraita and say: To prohibit rival wives of those with whom relations are forbidden, not the women themselves with whom relations are forbidden. The Gemara raises a difficulty: But he brought two proofs from the verse, both ''have intercourse with her'' and ''and consummate the levirate marriage with her.'' What, is it not because one phrase is required for the halakha of a woman with whom relations are forbidden and the other one for a rival wife?

The Gemara rejects this suggestion: No, this source and that source are necessary for a rival wife, and both phrases are required, for the following reason. One of them comes to prohibit a rival wife in a place where the mitzva of levirate marriage is applicable, and one serves to permit a rival wife in a place where the mitzva is not applicable. If a woman is the rival wife of one of those relatives forbidden to a given man, he is permitted to marry her following the death of her husband, as the prohibition proscribing rival wives of women with whom relations are forbidden pertains only to cases where the halakhot of levirate marriage are applicable, i.e., in the case of a brother’s wife.

The Gemara elaborates: What is the reason for this halakha? It is derived from the fact that the verse does not merely state: And consummate the levirate marriage, but instead emphasizes: ''And consummate the levirate marriage with her.'' This comes to teach: It is in a place where levirate marriage applies that a rival wife is prohibited; however, in a place where the levirate marriage is not applicable, a rival wife is permitted.

Rav Ashi said: The mishna is also precisely formulated in accordance with the opinion of Rava, as it teaches: Fifteen women exempt their rival wives, whereas the phrase: Are exempt and exempt others, is not taught. This indicates that the exemption of these women who are themselves forbidden relations does not necessitate any special inference, as this is not a novel halakha, and therefore the mishna does not even state this point explicitly. The novelty is that they exempt their rival wives. The Gemara summarizes: Conclude from this that Rava’s opinion is correct.

The Gemara asks: And if so, what is different about a forbidden relation that a special verse is not necessary to exclude it? The reason must be that a positive mitzva does not override a prohibition that entails karet. If so, a rival wife should also not require an additional verse, due to the fact that a positive mitzva does not override a prohibition that entails karet. The phrase ''to be a rival to her'' (Leviticus 18:18) teaches that the rival wife of a wife’s sister is prohibited like the wife’s sister herself.

Rav Aḥa, son of Beivai Mar, said to Ravina that this is what we say in the name of Rava: With regard to a rival wife too, a verse is not necessary, as is the case with a forbidden relation herself, since the prohibition in her case as well incurs karet, and therefore one would not think that the positive mitzva of levirate marriage overrides that prohibition. The verse is necessary