According to R"L that Yivum is again required in the case of a pregnant yevamah, would another brother be permitted to perform such Yivum, since the initial Yivum is invalid and apparently invalid min hatorah.
Thank You
Rashi here (35b DH d'Lo Sagi b'Lav Hachi) writes that if there was a Bi'as Me'uberes, the only way to free the Yevamah after she miscarries is through another Bi'ah, then Get. That is, the attempted Yibum must be consummated. (See Beis Meir EH 164:4 for the reason - which is apparently mid'Rabanan.) If so, another brother's Yibum won't help.
However, Rashi's explanation depends upon his Girsa in the Gemara. Rashi's Girsa was "d'Lo Sagi b'Lav Hachi ", while the other Rishonim - as well as the printed text of our Gemara - omit the last two words. Based on their text, the Rashba and Ritva learn that a Bi'as Me'uberes is ignored entirely according to Reish Lakish. If so, another brother can indeed do Yibum (and the brother who performed the Bi'as Me'uberes can do Chalitzah). The question of whether the Bi'as Me'uberes must be consummated is also reflected in the Sugya of 36b, by the way: "Rebbi Eliezer says she must be divorced with a Get" -- see the Rishonim there.
The Ritva here presents a second explanation for the Gemara (based once again on his Girsa) according to which the Yavam who attempted Yibum through a Bi'as Me'uberes should perform Yibum l'Chatchilah after the miscarriage (lest it appear that his Bi'as Me'uberes involved a transgression of the Isur Kares of Eshes Ach she'Lo b'Makom Mitzvah). According to this view, another brother should not perform Yibum l'Chatchilah .
(For a more detailed discussion of this issue, see Rav Aharon Yafen Zt'l's footnote #117 in the Mosad Harav Kook edition of the Ritva, Yevamos 36b.)
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf