Yevamot 26: Yibum Word Problems

Finishing chapter 2, and starting chapter 3, with a mishnah apiece. First, a case where a woman was able to leave her marriage - are any of the people involved in the case in court allowed to marry her? The answer - in sweeping terms for which there are of course exceptions - is no. Ulterior motives, and all. But with an important caveat or few. Plus, chapter 3 returns us to cases of yibum. For example, 4 brothers, two of whom married two sisters, where those two brothers then died, and the question is how yibum or chalitzah kicks in for the sisters. And the consequences when the parameters are changed, such as forbidden relationships messing with the yibum cases.