The folowing arises out of the contents of Insights 50b which I should spend more time on and which to me is difficult to grasp.
1. If a brother divorces his yevamah after making yibum
would a 3rd brother have to marry her? Or do we say
that once the zika relationship (to the 3rd brother) has
been removed through Shimon's yibum with Leah the latter
actually becomes asur to Levy as ashes achiv?
2. Could you please explain how a yevam could give a Get
to the Yevama? Wouldn't this not have to be after the
yevam lived with her? In such a case then, would this
not be a matter of divorcing a wife - not a yevamah?
1. Once Shimon did Yibum with her, the Mitzvah has already been fulfilled, and Levi does not need to do Yibum with her if Shimon then divorces her. Moreover, she would be prohibited to Levi, with Kares, because of "Gerushas Achiv" (since Shimon divorced her).
2. A "Get Yevamin" is a Takanah d'Rabanan which is able to invalidate the Yevamah from being able to do Yibum (to the brother who gave her the Get Yevamin, and to the other brothers) by giving her a Get. Nevertheless, she is still not permitted to marry anyone else until she does Chalitzah.
Similarly, if a Yavam did Ma'amar with the Yevamah (Ma'amar being a form of Kidushin with Kesef or with a Shtar, which the Rabanan instituted to be done with a Yevamah), the Ma'amar is cancelled through the giving of a Get Yevamin, and then Chalitzah is required to remove her Zikah and to permit her to marry anyone else.
Mordecai Kornfeld