Rebbi Eliezer says that a Yavam may annul the Neder of his Shomeres Yavam. (1)
Rebbi Yehoshua says that if there is only one Yavam he may annul the Neder of his Shomeres Yavam. (2)
Rebbi Akiva says that a Yavam may not annul the Neder of a Shomeres Yavam.
Rebbi Eliezer and Rebbi Yehoshua hold Yesh Zikah and Rebbi Akiva holds Ein Zikah. (3)
Beis Shamai holds that Ma'amar is a complete Kinyan. (4)
If the first Yavam does Ma'amar the second Yavam may invalidate her on the first Yavam by giving a Get or doing a Bi'ah.
Rebbi Elazar holds that even according to Beis Shamai Ma'amar is only Koneh to push off the Tzarah. (5)
If the Yavam is taken to Beis Din and he runs away he is obligated to provide Mezonos to the Shomeres Yavam.
Rabah asks if Ma'amar according to Beis Shamai makes an Erusin or Nisu'in.
A BIT MORE
1. Even if there is more than one Yavam.
2. Because Rebbi Yehoshua Yesh Zikah.
3. Zikah means that the Yevamah has the same attachment to the Yavam as an Arusah or a Nesu'ah.
4. Ma'amar means that the Yevamah received a Kidushin from the Yavam.
5. If two sisters are married to two brothers and one of the brothers dies and a third brother did Ma'amar to the Yevamah and subsequently the second Yavam dies the second Yevamah is free to get married without Chalitzah because she is the sister of the Ba'alas Ma'amar.
THE MITZVAH OF YIBUM
Rebbi Yehoshua says that if there is more than one Yavam the Neder of his Shomeres Yavam may not be annulled. The Rosh asks that even if there is more than one Yavam the principle Mitzvah is for the oldest to do the Yibum so why can't he annul the Neder as if there is no other Yavam? The Rosh answers that even though it is a Mitzvah l'Chatchilah for the oldest to do the Yibum, mid'Oraisa the younger one may also do the Yibum and if the oldest is overseas Beis Din forces the younger one to do the Yibum.
A Yavam may not annul the Neder of a Shomeres Yavam even if there is only one Yavam and even if he did Ma'amar. (Shulchan Aruch YD 234:7)
Because Ma'amar is only Koneh mid'Rabanan. (Shach)