Commentary for Yevamot 4:5

מצוה בגדול ליבם – as we derive from (Deuteronomy 25:6): “The first son [that she bears shall be accounted to the dead brother].”

Introduction Our mishnah answers the question, which of the surviving brothers must or can perform yibbum or halitzah.

The commandment of yibbum is upon the eldest [of the surviving brothers]. If he declines, they pass in turn to all the other brothers. If they decline, they return to the eldest and say to him, “the commandment is upon you; either perform halitzah or yibbum.” As we taught above in chapter two, mishnah eight, the duty of yibbum is incumbent upon the eldest surviving brother. The other brothers should not perform yibbum until their elder brothers have declined the opportunity. However, as we learned there, if a younger brother does perform yibbum before the older brother has a chance to accept or decline, his yibbum is valid. If the eldest brother does not want to perform yibbum, all of the other brothers are given the opportunity, in order of their age. If they all do not wish to perform yibbum, they (perhaps referring to a court) return to the eldest brother and tells him that he must either perform yibbum or halitzah. He is not allowed to leave this woman hanging any longer. The mishnah also teaches that the duty of halitzah is also primarily upon the eldest surviving brother.