I think the ending was pretty great. After thinking about it for a bit, I realize that the book has really highlighted how subjective my caring was.

(SPOILER)I cared for Anna, I rooted for Anna - I didn't really give much of a crap about Kate. Of course, I was rooting for someone to win a court battle, which would kill her sister.

Secondly, the major theme of the book is control. Every character is concerned with control in some way or another. The most obvious is Anna, of course, who wants control over her own body. But Kate wants control, to choose her death. Sara wants control, to keep Kate alive. Brian wants control, and though he is more pragmatic and accepting of Kate's eventual fate he is VERY determined to be the one who provides for his family. Jesse wants control and demonstrates it through his pyromania, something explicitly stated by Brian. The characters outside the family all have their own stuff in much the same vein; Campbell, Julia, etc. etc.

And then in the end Anna wins and it doesn't matter. Kate doesn't die. Some people do get what they want, or something approaching it, and most seem to come to terms with their situations. But the point is - the point of the whole book, in my eyes - that regardless of our desires and even our successes, we can't guarantee control.