daf bit: Yevamot 18
Tractate Yevamot is the first one in the order Nashim (Women). It covers
levirate marriages (among other topics). When a man dies without a child
his brother is supposed to marry his widow, with the goal of producing a
child in the deceased's name. The man can refuse, and there are cases where
he must refuse. (The woman can also refuse.)
The mishna on yesterday's daf (today's is 19) discusses the following case: there are two brothers, both married. One dies and the other then performs the levirate marriage (he now has two wives). A third brother is then born and the second dies. Now what? The woman who was twice widowed is exempt from doing this again because the third brother was not her (first) husband's contemporary; it appears that only brothers who already existed at the time of the marriage are part of this. What of the other widow, the second brother's first wife? She is also exempt because she is the other's rival (both were married to the same man). R. Shimon, however, says that the third brother may marry one of the wives, whichever he prefers. (18b)
The mishna on yesterday's daf (today's is 19) discusses the following case: there are two brothers, both married. One dies and the other then performs the levirate marriage (he now has two wives). A third brother is then born and the second dies. Now what? The woman who was twice widowed is exempt from doing this again because the third brother was not her (first) husband's contemporary; it appears that only brothers who already existed at the time of the marriage are part of this. What of the other widow, the second brother's first wife? She is also exempt because she is the other's rival (both were married to the same man). R. Shimon, however, says that the third brother may marry one of the wives, whichever he prefers. (18b)