Yevamot 30 - April 6, 5 Nissan

Today's daf is sponsored by Bernie & Susannnah Goldstein in gratitude for their family recovering from COVID. "לולי תורתך שעשועיי, אז אבדתי בעוניי. Daf Yomi during the pandemic has been a real source of strength. Thank you Rabbanit Michelle & Hadran team for making it accessible." Several different variations are brought in the Mishnayot of three brothers, each married to two sisters and a third wife, unrelated. In each situation one brother dies, another does yibum and then dies as well. The order of events is different as to who died first, was someone divorced at some point, etc. The Gemara tries to establish the differences between the cases that necessitated the Mishna to bring each one and what halakhot can be derived from each Mishna. When Rav states something that seems to be clear from the Mishsna, the Gemara infers that he must be teaching us something further, that once a woman is unable to do yibum with the brother at the moment she falls to yibum to him, she is never permitted to him, even if during the first "fall to yibum" her sister dies. From the first Mishna here, Rav Nachman infers that there is no zika, but from another Mishna, Rav Ashi infers that there is zika. How can each explain the other Mishna to fit with their approach? An inference from a Mishna here contradicts the Mishna on Yevamot 2b, as here it seems if the sister dies before the second wife marries her husband, she is not considered her co-wife, but in Yevamot 2, it's clear that as long as she dies before yibum, she is not considered a co-wife to be exempted in the case of erva on account of the other wife. Two resolutions are suggested. If there is a doubt about whether the wife who is erva is maybe betrothed or maybe not to the husband or maybe divorced, maybe not, his other wife (to whom he is certainly married) needs to do chalitza. Why?