QUESTION: The Beraisa states that a Ger who was conceived before his mother
converted ("Horaso sh'Lo b'Kedushah") and born after his mother converted
("Leidaso b'Kedushah") is prohibited from marrying his relatives from his
mother's side, such as his sister from his mother, or his mother's sister.
He may marry only his relatives from his father's side.
RASHI says that the reason for this Isur d'Rabanan is because if the Ger
would be permitted to marry his sister from his mother (since she was born
before his mother converted), then he might come to marry his sister from
his mother who was born *after* his mother converted, which would be an Isur
d'Oraisa of "Achoso."
However, Rashi earlier gives a different reason. We learned earlier (97b)
that Rav Sheshes prohibited the two sons of Yudan Amsa from marrying each
other's former wives (their sisters-in-law), even though the brothers were
both conceived and born before their mother converted. There, the reason for
the prohibition cannot be that they might come to marry a sibling from the
same mother after the Gerus, because since they themselves were born before
the Gerus, they would not be related to any children born after the Gerus!
Rather, the reason there, as Rashi there explains, is because people will
mistakenly think that if a Ger is permitted ot marry his brother's wife,
then a normal Jew may also marry his brother's wife. This reason -- that
people will confuse the laws of Arayos of a normal Jew with the Heter of a
Ger -- applies in our Gemara as well, so why did Rashi give a different
explanation in the Sugya here? (TOSFOS DH Nasa)
ANSWER: Rashi understood that the Isur in the Sugya here cannot cannot be
for the same reason as the Isur in the case of the sons of Yudan Amsa. The
Beraisa here is specifically discussing a Ger who was born after his mother
converted, "Leidaso b'Kedushah." The implication of the Beraisa is that a
Ger who was born *before* his mother converted would *not* be prohibited to
marry his relatives, and the reasoning that people might confuse the Ger
with a normal Jer does not apply to prohibit the Ger from marrying his
relatives. (Tosfos has difficulty explaining the wording of the Beraisa and
offers numerous solutions how to explain it, but Rashi's explanation avoids
the question entirely.)
Why, though, does that reasoning not apply here? A number of explanations
(a) RAV ELAZAR MOSHE HOROWITZ suggests that the reasoning that people might
confuse a Ger with a normal Jew is only a reason to prohibit a Ger from
marrying his relatives *l'Chatchilah* (such as the case on 97b). The Beraisa
here says that even b'Di'eved, if a Ger marries his sister, he must divorce
her. This stringency that he must divorce her is only because of a Gezeirah
that he might think that it is permitted to marry his sister who was born
*after* their mother converted.
(b) Rashi might hold that the reasoning that people will confuse a Ger with
a normal Jew cannot apply to *natural relatives*, such as one's sister.
Every Jew knows that it is forbidden to marry one's sister (c.f. Shabbos
145b). It is only a relative through marriage, such as one's sister-in-law
(Eshes Achiv), that is subject to error, whom a Jew might mistakenly think
that it is permitted to marry when he sees that a Ger may marry such a
relative. Thus, the reasoning for the Isur that Rashi mentions earlier (97b)
does not apply in the case of our Beraisa.
On the other hand, the Isur of our Beraisa -- a Gezeirah lest the Ger
confuse the Heter to marry a sister born before the mother converted with a
sister born after the mother converted, applies *only* to natural relatives.
The Rabanan did *not* institute such a Gezeirah for a relative through
marriage, as Rashi says later (98b, DH u'Mutar). Thus, the two different
Isurim complement each other.