Another yibum case - including when one brother dies before the other is born leads to a mnemonic how to keep this particular complication straight: "died, was born, does yibum" x2 - in a family of many brothers. And if the brother doing yibum dies... another brother should be taking it on. Yaakov is brought as an example - a strawman, and with echos of the biblical Yaakov. Also, when a woman exempts her co-wives, and other cases when she doesn't - ie, it's not "across the board" and is contingent on a legitimate marriage. Plus, how halitzah has impact on the yibum relationship. Plus, more on the Levi/Rebbi complaint/defense about the 15 cases in the opening mishnah.