Apparently, this mishna indicates that his wife’s sister, whether from the father, i.e., a paternal sister, or from the mother, i.e., a maternal sister, is forbidden. From where do we derive this halakha that the prohibition applies even to his wife’s maternal sister? The Gemara responds: It is derived from the prohibition proscribing his sister. Just as his sister is forbidden whether she is his sister from his father or from his mother, so too here, a wife’s sister is forbidden whether from the father or from the mother.

The Gemara challenges the validity of this source: And let it be derived from the halakha with regard to his aunt: Just as the prohibition with regard to his aunt applies only to the wife of his father’s brother from his father but not from his mother, i.e., the wife of his father’s paternal brother but not the wife of his father’s maternal brother, so too here, the prohibition with regard to his wife’s sister should apply only to her sister from her father but not to her sister from her mother. The Gemara answers: It is reasonable that he should derive the halakha in this case from the case of his sister, as the tanna thereby derives the halakha of one’s own relatives from another case of his own relatives, whereas his aunt is forbidden as his father’s relative.

The Gemara counters: On the contrary, he should derive the halakha in this instance from the case of his aunt, as he thereby derives the halakha in a matter prohibited through betrothal from another matter prohibited through betrothal. The Gemara concludes: Rather, the halakha of a wife’s sister is derived from that of a brother’s wife, as they are both something forbidden by means of betrothal and they are his own relatives.

The Gemara asks: And in the case of a brother’s wife itself, from where do we derive that the prohibition applies to the wife of both a paternal and a maternal brother? As it is taught in a baraita: “You shall not uncover the nakedness of your brother’s wife” (Leviticus 18:16), which indicates: Whether from the father or from the mother.

The baraita elaborates: Do you say the prohibition applies whether she is the wife of one’s brother from his father or from his mother, i.e., whether she is the wife of one’s paternal brother or maternal brother? Or perhaps it is only the wife of one’s brother from his father and not from his mother? It may be inferred logically from the fact that the Torah rendered him liable here and rendered him liable with regard to his sister: Just as with regard to his sister he is liable to receive punishment whether she is his sister from his father or from his mother, so too here, he is liable to receive punishment whether she is the wife of his brother from his father or from his mother.

Or perhaps go this way and compare this case to the case of an aunt. The Torah rendered him liable here and rendered him liable with regard to his aunt, i.e., the wife of his father’s brother: Just as with regard to his aunt, he is liable to receive punishment only for the wife of his father’s brother from his father and not for the wife of his father’s brother from his mother, so too here, he is liable to receive punishment only for the wife of his brother from his father and not from his mother.

The Gemara analyzes these two possibilities: Let us see to which case it is more similar. We should derive the halakha with regard to his own relatives, i.e., his brother’s wife, from another case of his own relatives, i.e., his sister, and the halakha with regard to his aunt cannot be used to prove otherwise, as she is his father’s relative. Or perhaps go this way: We should derive the halakha of a matter prohibited through betrothal, i.e., a brother’s wife, from another matter prohibited through betrothal, i.e., his father’s brother’s wife, and the halakha with regard to his sister cannot be used to prove otherwise, as it is a prohibition that comes on its own.

Since it is impossible to prove which halakha should serve as the model for the case of a brother’s wife, the verse states a second time in the same verse: “It is your brother’s nakedness” (Leviticus 18:16), in order to emphasize that she is forbidden whether she is the wife of his brother from his father or from his mother.

The Gemara challenges this interpretation of the extra phrase in the verse: Say that both this first part of the verse and that latter part refer to the wife of a brother from the father, i.e., the wife of one’s paternal brother. As for the repetition, one part of the verse renders prohibited a woman who has children, who is prohibited from marrying her husband’s brother during her husband’s lifetime even after they are divorced, and the other one renders prohibited a woman who does not have children, who is also prohibited from marrying her husband’s brother during her husband’s lifetime, even after she is divorced. The Gemara responds: The prohibition proscribing a woman who does not have children for the duration of her husband’s lifetime is derived from the statement of Rav Huna (54b), and it requires no further source.

The Gemara presents another challenge: Say that both this first part of the verse and that latter part refer to the wife of a brother from the father, i.e., the wife of one’s paternal brother. As for the repetition, one part of the verse renders a woman who has children prohibited from marrying her husband’s brother for the duration of her husband’s lifetime, even after she is divorced, and the other one renders prohibited a woman who has children and indicates that it is prohibited for her to marry her husband’s brother even after her husband’s death.

The Gemara responds: The halakha that it is prohibited for a woman who has children to marry her husband’s brother even after her husband’s death does not require a verse. From the fact that the Merciful One says that one who has no children is permitted to marry the brother after her husband’s death, it can be inferred that if she has children, it is prohibited for her to marry her husband’s brother.

The Gemara raises another challenge: But perhaps if she does not have children it is prohibited for her to marry everyone else and permitted for her to marry her yavam, as specified by the Torah. However, if she does have children it is permitted for her to marry everyone and it is permitted for her to marry her yavam as well. Alternatively, perhaps if she does not have children it is a mitzva for her husband’s brother to marry her, and if she does have children it is optional. Consequently, the extra phrase in the verse should be necessary in order to indicate that the prohibition against marriage applies in all of these cases.

Alternatively, if she does not have children, yes, she is permitted to her husband’s brother, and if she does have children, no, she is not permitted to him, but the punishment of karet does not apply to this prohibition. This is because the source of the prohibition is the mitzva for them to marry after the husband’s death, and a prohibition that stems from a positive mitzva has the status of a positive mitzva, but nothing more. Consequently, the extra phrase in the verse should be necessary in order to indicate that the punishment of karet is still applicable.

The Gemara responds: The Torah wrote a different verse to teach that the prohibition against marrying one’s brother’s wife is in full force, including the punishment of karet, in these cases: “He has uncovered his brother’s nakedness; they shall be childless” (Leviticus 20:21). Therefore, the extra phrase mentioned in the baraita can indicate that the prohibition against marrying one’s brother’s wife applies both to the wife of a paternal brother and to the wife of a maternal brother.

The Gemara suggests: Say that the halakha of the wife of a brother from one’s mother should be just like that of the wife of a brother from his father: Just as the wife of a brother from one’s father is permitted, i.e., one can marry her, after her husband’s death, so too, the wife of a brother from his mother should be permitted after her husband’s death. The Gemara responds: The verse states: “She is your brother’s nakedness,” thereby emphasizing that she shall remain after her husband’s death as she is during his life.

§ The Gemara above stated that all of the instances of forbidden intercourse are compared to one another. Therefore, since there is already a verse that 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), the Gemara asks: Why do I need the phrase written with regard to one’s sister that states that intercourse is punishable by karet?

The Gemara answers that it is necessary for that which Rabbi Yoḥanan said, as Rabbi Yoḥanan said that if he performed all of them, i.e., he violated every prohibition against engaging in forbidden intercourse in a single lapse of awareness, he is liable to receive punishment for each and every one. One might have thought that there is one prohibition against engaging in forbidden intercourse, and many different ways to violate the prohibition. Therefore, if one violated the prohibition in different ways during one lapse of awareness, one is liable to bring only one sin-offering. This verse therefore indicates that each act of intercourse with a relative with whom intercourse is forbidden is a violation of an independent prohibition.

The Gemara asks: And according to Rabbi Yitzḥak, who said that all forbidden relations for which one is liable to receive karet were included in the verse: “The souls that do them shall be cut off from among their people,” why was the punishment of karet with regard to one’s sister singled out? In order to sentence him to karet and not to flogging. Although he has violated a prohibition, which generally carries a punishment of flogging, he is not flogged due to the fact that he is liable to receive the more severe punishment of karet. Since he has used the verse to teach this halakha, from where do we know to divide the prohibitions against engaging in forbidden intercourse and consider each an independent prohibition?

The Gemara answers: He derives it from the verse “And you shall not approach a woman to uncover her nakedness as long as she is impure by her uncleanness” (Leviticus 18:19), which serves to render him liable to receive punishment for each and every woman.

The Gemara poses another question: Why do I need the phrase that the Merciful One writes with regard to one’s aunt, which states: They shall be childless? The death of one’s children is included in the punishment of karet, and it has already been established that all of the forbidden intercourse is punishable by karet. The Gemara answers that it is necessary for that which Rabba said, as Rabba raised a contradiction: It is written with regard to one who had intercourse with his brother’s wife: “They shall be childless” (Leviticus 20:22), and it also states with regard to one who had intercourse with his aunt: “They shall die childless” (Leviticus 20:21). How so? If he already has children, he will eventually bury them; if he does not have children, he will go childless.

The Gemara comments: And it is necessary to state: They shall be childless, and it is also necessary to state: They shall die childless. As, if the Merciful One had written only: They shall be childless, I would have said that only those children he had before his sin will die, but those born to him from the time of his sin and on, no, they will not die. The Torah therefore states: They shall die childless, indicating that even if he has children afterward they will die and he will remain childless. And similarly, if the Merciful One had written only: They shall die childless, I would have said that this is referring to children born from the time of his sin and on, but those born from the beginning, before he sinned, no, they will not die. It is therefore necessary to mention both expressions.

§ The Gemara above (54a–54b) derived that the initial stage of intercourse is considered an act of sexual intercourse with regard to prohibitions that are punishable by capital punishment or karet. The Gemara asks: From where do we derive that the initial stage of intercourse is considered an act of sexual intercourse with regard to those liable to receive punishment for violating ordinary Torah prohibitions?

The Gemara answers: From the fact that the Merciful One reveals with regard to a designated maidservant that the prohibition has been violated only through an act of cohabitation with seed, i.e., a complete act of sexual intercourse, in the verse “And whoever lies with a woman in cohabitation with seed, and she is a bondmaid designated to a man” (Leviticus 19:20), by inference, those liable to receive punishment for violating ordinary prohibitions are liable even through the initial stage of intercourse.

The Gemara responds: On the contrary, from the fact that the Merciful One reveals that the initial stage of intercourse is considered sexual intercourse with regard to forbidden relationships for which one is liable to receive karet, then, by inference, those liable for violating ordinary prohibitions are liable only through the completion of the act of sexual intercourse and not merely for the initial stage of intercourse. Rav Ashi said: If so, let the verse remain silent in the case of a designated maidservant, and it would be assumed that one is liable to receive punishment only for completing the act of sexual intercourse. The fact that the Torah specified that in this case one is liable to receive punishment only for completing the act of sexual intercourse indicates that with regard to other ordinary prohibitions one is liable even for the initial stage of intercourse.

The Gemara asks: From where do we derive that the initial stage of intercourse is considered an act of sexual intercourse with regard to those liable to receive punishment for violating prohibitions specific to the priesthood? Since these prohibitions are unique in that they apply only to priests, their parameters cannot be derived from prohibitions that apply to the entire population. The Gemara answers: It is derived from a verbal analogy between the words taking and taking. This verb appears in prohibitions punishable by karet, e.g., “And if a man shall take his sister” (Leviticus 20:17), and in prohibitions of the priesthood, where it states: “They shall not take a woman that is a harlot” (Leviticus 21:7).

The Gemara asks further: From where do we derive that the initial stage of intercourse is considered an act of sexual intercourse with regard to those liable to receive punishment for violating a positive mitzva, e.g., one who has intercourse with an Egyptian or Edomite convert? The verse states: “The children that are born to them of the third generation may enter into the assembly of the Lord” (Deuteronomy 23:9). It is therefore a positive mitzva that only the grandchildren of these converts may have intercourse with a Jew.