Yevamot 24A

התם, אי למאן דאמר יש זיקה ⁠— יש זיקה, ואי למאן דאמר אסור לבטל מצות יבמין ⁠— אסור לבטל מצות יבמין. הכא ⁠— כל חד וחד אימור דידיה קא מתרמיא ליה.

There, in that Mishnah, if it is according to the one who said that the levirate bond is substantial, then there is a bond in that case, as two sisters were certainly married to the brothers and require levirate marriage. And if it is according to the one who said that it is prohibited to nullify the levirate mitzva through marrying the sister of the yevama, then the explanation of that Mishnah is that it is prohibited to nullify the levirate mitzva and for this reason they must perform ḥalitza and may not enter into levirate marriage. However, here, in this Mishnah, there is uncertainty concerning the betrothal such that with regard to each one of the brothers, one could say that possibly he encountered his own yevama. Perhaps each brother took his own brother’s wife in levirate marriage, and for this reason the rabbis did not issue a decree.

קדמו וכנסו אין מוציאין כו׳. תני שילא: ואפילו שניהם כהנים. מאי טעמא ⁠— חלוצה דרבנן היא, וספק חלוצה לא גזרו בהו רבנן.

§ The Mishnah states that if they married their wives before consulting the court, the court does not remove them from the marriage. Sheila teaches a baraita that clarifies the Mishnah: And this is the case even if both of the brothers were priests. A woman who performed ḥalitza is normally forbidden to a priest, yet in this case, although the brother of the other man performed ḥalitza, they are not required to divorce. What is the reason for this halakha? It is as follows: A ḥalutza is forbidden to a priest by rabbinic law because her status is similar to that of a divorcée, who is forbidden to a priest by Torah law. And in a case of uncertainty as to whether she is a ḥalutza, since it may not have been her yavam who performed the ceremony, the rabbis did not issue a decree.