Yevamot 18 - Bond of yibum
When a brother dies childless, his wife is bound to the other brother: she cannot marry out of the family. The remaining brother either does a yibum (marries her) or gives her a chalitzah (releases her). This bond thus is similar to marriage.
How strong is it? Do we say that it is almost like marriage that already exists? Take, for example, this case: a woman, whose husband died, is awaiting a yibum, but then she dies. Can the brother marry this woman's mother? If the brother and the woman were actually married, then of course he cannot marry also her mother. But they were never married - they were only connected through a yibum-bond.
Rav Huna says that yes, he can marry her mother. There is no strong connection created by the yibum bond. Now the Talmud begins to analyze the statement. Why didn't Rav Huna simply state, "There is no bond!?" - Because if he did that, we might have thought that even while the woman (yevamah) is still alive, he can disregard the bond and marry her mother. This is not so, however: one is forbidden to destroy the potential for the mitzvah of yibum, and that is what he would have done by marrying the mother.
Rav Yehudah disagrees and says that one cannot marry the mother in this situation. Same question: why didn't Rav Yehudah simply state that "there is a bond?!" - He wants to teach us another lesson. If he said, "There is a bond," we might have thought that it is true only when there is only one remaining brother, and that is why the bond is strong. But should there be many brothers, and the power of the bond be divided among them, then perhaps it would not stop a brother from marrying the woman's mother - so Rav Yehudah had to state this prohibition for all brothers.
Art: Portrait Of The Artists Mother by Franz Marc