QUESTION: The Gemara cites the verse, "Ervas Achicha Hi" (Vayikra 18:16), which teaches that one is Chayav Kares for having relations with "Eshes Achiv" (one's brother's wife) even when he and his brother share only the same mother ("Eshes Achiv Min ha'Em").
However, that verse is written in the Parshah which discusses the Lo Ta'aseh of having relations with various Arayos (Parshas Acharei Mos); it does not openly prescribe the punishment of Kares for "Eshes Achiv." Consequently, it could be argued that the verse "Ervas Achicha Hi" includes "Eshes Achiv Min ha'Em" only in the Lo Ta'aseh but not in the punishment of Kares.
TOSFOS (2a, DH Eshes Achiv) points out that the Mishnah in the beginning of Yevamos (2a) lists "Eshes Achiv Min ha'Em" as one of the Arayos which exempt a woman from Yibum, and the Gemara (3b) states that all of the Arayos which exempt a woman from Yibum are punishable with Kares. Accordingly, the Mishnah here supports the assertion of the Gemara when it says that "Eshes Achiv Min ha'Em" is Chayav Kares.
Tosfos asks that other sources imply that there is no Kares for "Eshes Achiv Min ha'Em." The Toras Kohanim (Kedoshim 11) and the Yerushalmi (Shabbos 7:2) state that the verse which discusses the punishment of Kares for the Isur of "Eshes Achiv" (in Parshas Kedoshim) adds the words "Nidah Hi" in order to teach that the Isur of "Eshes Achiv" is punishable with Kares when it is similar to the Isur of Nidah; just as the Isur of Nidah has a Heter (she can become Tahor), so, too, the type of "Eshes Achiv" for whom one is Chayav Kares is that which has a Heter, or "Eshes Achiv Min ha'Av" (the Heter being a situation of Yibum). This implies that only "Eshes Achiv Min ha'Av" -- and not "Eshes Achiv Min ha'Em" -- is included in the punishment of Kares.
Although the Toras Kohanim and Yerushalmi may disagree with the Gemara's interpretation of "Nidah Hi," they certainly cannot disagree with the Mishnah in the beginning of Yevamos which states that "Eshes Achiv Min ha'Em" is exempt from Yibum because it bears a punishment of Kares. How do the Toras Kohanim and the Yerushalmi understand the Mishnah?
(a) TOSFOS (2a) and the RAMBAN (54b) suggest that perhaps the Toras Kohanim means the same thing as the Gemara here says (on 54b). The Gemara derives that "Eshes Achiv Min ha'Av" -- in a case in which the brother was childless but divorced his wife -- is punishable with Kares despite the fact that the brother would have been permitted to marry the woman in a situation of Yibum (i.e. had her husband died childless). This is derived from the words "Nidah Hi," which imply that the verse applies to the type of "Eshes Achiv" which has a possible Heter (she is like a Nidah who can become permitted). The Torah teaches that even "Eshes Achiv Min ha'Av" will sometimes bear a punishment of Kares.
Perhaps this is the intention of the Toras Kohanim as well. The Toras Kohanim does not mean that the verse refers only to "Eshes Achiv Min ha'Av," but that the verse refers even to "Eshes Achiv Min ha'Av." "Nidah Hi" teaches that although the "Eshes Ach" has a Heter (when the brother dies and his wife falls to Yibum), the Chiyuv Kares remains when the brother divorces his wife.
(b) Tosfos here (2a) and in Kerisus (14b) rejects this explanation based on the wording of the Yerushalmi and the Toras Kohanim and suggests a different explanation. The Toras Kohanim and Yerushalmi do not mean to exempt "Eshes Achiv Min ha'Em" from Kares. Rather, they derive from the words "Nidah Hi" that "Eshes Achiv Min ha'Em" is not punished with Ariri, whereas all of the other Arayos are punished with Ariri. (Although the verse mentions Ariri only with regard to "Dodaso" (one's aunt) and "Eshes Achiv," the Hekesh of Rebbi Yonah -- which compares all of the Arayos to each other -- teaches, from "Dodaso," that all of the Arayos bear the punishment of Ariri.)
(Perhaps the logic behind this distinction is as follows. When the brother sins and lives with the widow of his brother (from his mother), he might have pure intentions to "build the house" of his deceased brother by bearing children with his widow. Therefore, the Torah "rewards" him measure for measure and does not punish him with the death of his children. -M. Kornfeld.)
Tosfos adds that this explanation is tenable only if Ariri and Kares are different punishments. Kares refers to "early death" while Ariri adds the aspect of dying childless. This is not the view of RASHI here (DH Aririm).
If Kares and Ariri are different punishments, however, what is the Gemara's intention when it asks why does the Torah say "Aririm Yamusu" in the verse of the Isur of "Dodaso" (Vayikra 20:20; see RASHASH here)? The Torah must write "Aririm" to teach the additional punishment of Ariri! The answer is that the Gemara wants to know why the verse says "Aririm" twice, once in the verse of "Dodaso" and a second time in the verse of "Eshes Achiv" (Vayikra 20:21). The Torah could have mentioned "Aririm" once, and the Hekesh of Rebbi Yonah would have taught that all of the other Arayos are also punishable with Ariri.
(c) A third possible approach (not mentioned by the Rishonim) is that the Yerushalmi actually maintains that "Eshes Achiv Min ha'Em" is exempt from Kares. The Yerushalmi maintains that although there is no Kares for "Eshes Achiv Min ha'Em," that relationship still exempts a person from Yibum because it is included in the Hekesh of Rebbi Yonah which teaches which women are exempt from Yibum (as Tosfos explains on 3b, DH Mah Achos). That Hekesh is written in Parshas Acharei Mos, where "Eshes Achiv Min ha'Em" is included (through the words "Eshes Achicha Hi"), as the Gemara here concludes. (Although the verse which compares all of the Arayos with each other according to Rebbi Yonah (54b) specifically mentions Kares, "Eshes Achiv Min ha'Em" is exempt from Kares because, according to the Yerushalmi, another verse ("Nidah Hi") excludes "Eshes Achiv Min ha'Em" from Kares.)
The Acharonim adduce proof from another statement that the Yerushalmi indeed maintains that "Eshes Achiv Min ha'Em" is not punishable with Kares. The Mishnah in Kesuvos (29a) discusses which type of young woman (Na'arah) is entitled to receive reparation from a man who rapes her. One of the women mentioned in the Mishnah is "Eshes Achiv... even though she is forbidden with an Isur Kares." The Yerushalmi (cited by TOSFOS there, DH v'Al Eshes Achiv) asks how is it possible for a fine to be levied when a man rapes "Eshes Achiv"? The rapist must pay a fine only when the Na'arah was a Besulah and was not married at the time of the rape (if she was married, the rapist is punished with Misah and not with a monetary fine). If the Na'arah was married to his brother but never had relations with him (and thus she is still a Besulah), and his brother died childless (and thus he is not punished with Misah for transgressing the Isur of "Eshes Achiv"), the rapist is not prohibited to the Na'arah at all! On the contrary, he is obligated to do Yibum with her, and thus there should be neither a punishment of Kares nor a fine! The Yerushalmi answers that the Mishnah refers to a case in which the brother who died had children from another wife, and thus his wife (the Na'arah) does not fall to Yibum.
There seems to be a much more obvious answer to the Yerushalmi's question. The Mishnah simply may refer to an "Eshes Achiv Min ha'Em"! Although her husband died childless, there is no obligation of Yibum with her (since the brothers do not share the same father), and thus the brother is obligated to pay a fine for raping her. Why does the Yerushalmi not suggest this answer? (TOSFOS CHADASHIM on the Mishnayos; this indeed is how the RIF in Kesuvos answers the Yerushalmi's question. See also RASHASH there.)
If the Yerushalmi maintains that "Eshes Achiv Min ha'Em" bears no punishment of Kares, then its question is easy to understand. The Mishnah cannot refer to "Eshes Achiv Min ha'Em" because it clearly states that the "Eshes Achiv" which it discusses is punishable with Kares.
(The RIF and others suggest that the "Eshes Achiv" of the Mishnah may refer to his brother's divorcee and not his widow. Perhaps the Yerushalmi maintains that this cannot be the case of the Mishnah because only later (Kesuvos 38a) does the Mishnah there introduce the laws of raping a divorcee. Moreover, one Tana there maintains that there is no monetary penalty at all in the case of the rape of a divorcee. Although all of the commentaries (see Rashi there) equate the laws of a divorcee to those of a widow, it is possible that the Yerushalmi understands otherwise. The Yerushalmi may maintain that all of the Tana'im agree that there is a monetary penalty for a widow because she returns to her father's home after she is widowed; see the Mishnah in Nedarim 70a and 71a.)