Yevamos 2:1-2

Yevamos 2:1

The case of the wife of a deceased brother who was not alive at the same time as the surviving brother (in mishna 1:1) is that there are two brothers and one of them died, after which a third brother was born. The older surviving brother (who was alive at the same time as the deceased brother) then married the deceased’s widow through yibum and subsequently died. This woman is exempt from yibum with the youngest brother because she is the wife of a brother who was not alive at the same time, and her co-wives are likewise exempt. If the middle brother married her with a contract but did not consummate the marriage before he died, then the co-wife performs chalitzah but not yibum.

Yevamos 2:2

Let’s say that there were two brothers, one of whom died and the other of whom married his widow via yibum, then a third brother was born and the second brother died. [The difference between this mishna and the previous one is that in 2:1 the third brother was born before the second brother performed yibum.] This woman is exempt from yibum with the youngest brother because she is the wife of a brother who was not alive at the same time, and her co-wives are likewise exempt. If the middle brother married the widow with a contract but did not consummate the marriage before he died, then the co-wife performs chalitzah but not yibum. Rabbi Shimon says that in this scenario the youngest brother may perform yibum or chalitzah with any of the wives. (Since the second brother married the widow before the youngest brother was born, she was already the second brother’s wife and no longer the wife of a deceased brother who was not alive at the same time.)