Levirate Marriage and the Family[ ]daughter, grandmothers-in-law, and stepmother-in-law. It omits men-
tion of several women, because they would also have been forbidden to
the deceased brother and thus would never be potential levirate part-
ners.^57 The rule stated in Mishnah Yevamot :, however, extends the
prohibition against a union far beyond those women. It posits that if A’s
brother were married to A’s daughter or another woman who is forbid-
den to A, and at the same time had another wife who had no relation-
ship to A beyond being his brother’s wife, that brother’s death without
children would not necessitate levirate marriage. A cannot marry his
daughter, nor, according to the Mishnah, can he marry her “co-wife,”
his brother’s other widow.
On the surface, this prohibition seems surprising. The co-wife in
question, after all, is not related by blood to A. She is related to him by
marriage, but that, of course, is always true when levirate marriage is
mandated! Why then should A be forbidden to perform levirate marriage
or halitza with this woman?
Responding to the opening assertion of the mishna, “Fifteen women
exempt their co-wives and the co-wives of their co-wives from halitza
and levirate marriage,” the Bavli asks:
a. From whence [in the Torah] do we learn this?
b. As our rabbis taught: “Do not marry a woman as a rival to her
sister and uncover her nakedness in the other’s lifetime” (Lev.
18:18). Why is the word aleha used?
c. Because it says, “Her brother-in-law shall unite with her
(a leha)” (Deut. 25:5) — I might deduce from this that the verse is
[mandating levirate] even with any one of the relatives forbidden
by the Torah, [so] here [in Leviticus] it says “aleha” and there
[in Deuteronomy] it says “aleha” — just as there it is a religious
duty, so too here [even though] it is a religious duty. {Even if the
union between a man and his wife’s sister was a levirate union, a
religious duty, nonetheless it would be forbidden.}
d. The Torah also says “Do not marry [a woman as a rival to her
sister]” — this would only prohibit her, from whence do we learn
that her co-wife (t za ra) too [is prohibited]? The Torah says, “as
a rival” (litzror).... From here the sages teach: Fifteen women
exempt....^58