According to the Girsa of the Rif and other Rishonim, Rebbi Elazar says "*in
all of the cases* ("b'Chulan") we instruct the Ketanah to do Mi'un." Is
Rebbi Elazar actually arguing that in *all* of the cases of the Mishnah, the
Ketanah should do Mi'un? The Mishnah discusses not only cases of a Ketanah
and a Gedolah who are Tzaros, but also a Ketanah and a Chareshes who are
Tzaros. In the case of a Ketanah and a Chareshes, where the Yavam lived with
both of them, does Rebbi Elazar also hold that we instruct the Ketanah to do
Mi'un so that the Chareshes will not be Asur to the Yavam?
(a) The RA'AVAD on the Rif and on the Rambam (Hilchos Yibum 5:24) writes
that we do not instruct a Ketanah to do Mi'un when her Tzarah is a
Chareshes. He explains that the Gemara's reason for instructing a Ketanah to
do Mi'un is because "Gedolah Ramya Kamei" -- by doing Mi'un, the Ketanah
enables the Gedolah to fulfill her Chiyuv d'Oraisa of Yibum. In contrast, a
Chareshes has no Chiyuv d'Oraisa to do Yibum, and therefore we do not
instruct the Ketanah to do Mi'un just so that the Chareshes can do Yibum.
(b) The RAMBAN and other Rishonim disagree with the Ra'avad. The Ramban says
that the only reason that the Gemara (109b) gives to "distance oneself from
Mi'un" is because after the Ketanah grows up she will regret doing Mi'un and
will be left without a husband. In the case of the Mishnah, though, where
the Yavam lived with the Chareshes after living with the Ketanah, the
Ketanah becomes Asur to the Yavam in any case (and she will have to do
Chalitzah and receive a Get if she does not do Mi'un)! Therefore, there is
no point in discouraging her from Mi'un.
Why, then, does the Gemara suggest that perhaps Rebbi Elazar only stated his
ruling in the case of the previous Mishnah but not in the case of our
Mishnah? And how could we suggest that he taught it only in the case of our
Mishnah and not in the previous Mishnah, where the sister of the Ketanah who
does Mi'un falls to Yibum to the Ketanah's husband? In any case, if the
Ketanah does not do Mi'un in these cases she will be sent away with a Get,
so we should certainly instruct her to do Mi'un! And why does the Tana Kama
argue with Rebbi Elazar in these cases?
The RAMBAN explains that in our Mishnah, perhaps we should penalize the
Yavam for living with the Chareshes (when it was prohibited to do so) after
living with the Ketanah, and that is why we should not let the Ketanah do
Mi'un. In the previous Mishnah, it might be preferable not to instruct the
Ketanah to do Mi'un because we *do not want* her sister to fall to Yibum
after her Mi'un. If her sister falls to Yibum after her Mi'un, it will look
like the Yavam is marrying his "Achos Ishah" (since not everyone will
realize that the Ketanah did Mi'un).
Therefore, when a Ketanah and Chareshes fall together to Yibum, and the
Yavam does Yibum with the Ketanah and afterwards he also lives with the
Chareshes, the Ketanah should do Mi'un so that it is as if she never fell to
him, and the Chareshes will be permitted to the Yavam.
(c) The RAMBAM (Hilchos Yibum v'Chalitzah 5:28) rules that if the Yavam
lived with the Chareshes and then the Ketanah, we do instruct the Ketanah to
do Mi'un and to uproot the Zikah of Yibum from herself, leaving the
Chareshes as the only Yevamah -- like the Ramban said. However, he adds that
even after the Mi'un, the Chareshes should be divorced with a Get.
The RAMBAN, RASHBA and other Rishonim are dismayed with this ruling. If the
Ketanah retroactively is not a Tzarah of the Chareshes (because of her
Mi'un), then why does the Yavam need to divorce the Chareshes with a Get?
One might answer that even the Mi'un of the Ketanah does not help to permit
the Chareshes, because at the time she fell to Yibum it certainly looked
like the Ketanah was also a Yevamah, and therefore the Rabanan enacted that
Bi'ah with her is Posel the Chareshes even after Mi'un. This cannot be the
answer, though, because the Rambam himself (ibid. 5:30) writes that in a
case where the Yavam lived with a Ketanah and then with a Gedolah, the
Ketanah should do Mi'un and then the Gedolah retroactively becomes the only
Yevamah and *is* permitted to the Yavam! If we suggest that Mi'un does not
completely permit the other Yevamah (as in the case of the Chareshes), then
we should say that the Gedolah is *not* permitted, because she became Pasul
through the Yavam's Bi'ah with the Ketanah that preceded his Bi'ah with her.
Because of this problem, the Rishonim reject the ruling of the Rambam.
The VILNA GA'ON (EH 171:13) points out that the Rambam himself answers this
question. The answer is indeed that the Rabanan enacted that the Bi'ah of
the Ketanah before Mi'un is Posel the Chareshes, as suggested above.
However, in the case of the Gedolah and Ketanah, where the Yavam lived with
the Ketanah and then with the Gedolah, the Yavam may remain with the Gedolah
because, as the Rambam writes, the Kinyan of the Gedolah is a "Kinyan Gamur"
(i.e. a Kinyan d'Oraisa). The Rabanan did not enact that Bi'ah before Mi'un
can disqualify a "Kinyan Gamur." It can only disqualify a Kinyan that is not
complete (i.e. a Kinyan d'Rabanan), such as the Kinyan of a Chareshes.