Yevamos 109a — Rav Soloveitchik Fascinating Vort : How Can Nisuin Create Yibum?

The Gemara asks: מיתה מפלת או נישואין הראשונים מפילין. Rav Soloveitchik points out that נישואין הראשונים מפילין is extremely difficult to understand. How can the marriage itself be the cause of yibum? Those very נישואין are what create the issur of אשת אח. Before she married his brother, she was permitted to the future yavam. The marriage is what makes her assur. So how can the same marriage that creates the issur also be what creates the future נפילה to yibum? Rav Soloveitchik explains that yibum must somehow already be built into the original אישות. The marriage does not only create איסור אשת אח; it also creates the framework that can later produce היתר יבום if the husband dies childless. He sees this in the Rambam’s language regarding a woman who was divorced and then remarried: הרי חזרה להיתרה How can remarriage be called a “return to heter” when remarriage actually makes her אשת אח again? The answer is that she has returned to an אישות that carries within it the potential for yibum. That is the depth of the sugya: the very same נישואין that create the prohibition may also contain the future היתר.