Refuted Explanation: Rashi explains because the Torah writes "u'Be'alah". implying that she must be fit for Bi'ah.

Refutation: But Chayvei La'avin are not fit for Bi'ah, yet they are subject to Kidushin?

Authentic Explanation: The explanation of 'Ein Mesurin le'Bi'ah therefore is that the Isur Bi'ah is caused by the Kidushin, where (for example) he betrothed one of two sisters S'tam and ...

Authentic Explanation cont.): Where, before the Kidushin, each sister was permitted, and both of whom became forbidden on account of the Kidushin ...

Authentic Explanation (concl.): Whereas the Isur Bi'ah of Chayvei La'avin was not caused by the Kidushin, seeing as they were forbidden already beforehand just as they were afterwards.

Clarification: This refers to the single brother, and as for the two, one performs Chalitzah and one, Yibum.

Clarification: Rashi explains - 'Specifically, Chalitzah first and then Yibum' - meaning that the single one must do Chalitsah first, before one of the two performs Yibum after his brother has performed Chalitzsah, since Mah Nafshach it is permitted ...

Reason: Because if his brother made Chalitzah with the Yevamah, the Zikah has been removed, and if he did not, then he has performed Yibum with her ...

Clarification (cont.): But for one of the two to perform Yibum first, even after his brother has done Chalitzah, in which case there is no problem of Achos Zekukaso as Tosfos explained, is not permitted ...

Reason: Because since the single brother did not do Chalitzah with both of them, one must suspect that the one with whom he performs Yibum is in fact, the Yevamah of the latter ...

Reason (cont.): And it transpires that he (the former) will be guilty of 'marrying' a Yevamah le'Shuk who has not performed Chalitzah.

Refuted Explanation: And we cannot explain 'Specifically, Chalitzah first and then Yibum - with reference to the two brothers - but not the other way round', - because he may transgress Achos Zekukaso ...

Refutation: Because that we already know from the Reisha.