Her brother died young in the war if I remember correctly, so there might be some of that in there. Though I, too, kind of did an "awww" when she just dumped it on us that he died.
Atticus makes sense to me. Think back to the lynching scene in TKAM. Afterward he says that Mr. Cunningham is a good guy. No he's not. He was going to kill a black man that OBVIOUSLY didn't rape Mayella. OK, sure, Mr. Cunningham is the lone holdout on the jury, but that's only because of Scout and Atticus and not anything to do with Tom. And think back to Mrs. Dubose. Sure, she is a morphine addict and she kicks it right before she died just because. That takes some guts. And when I teach the book, I use her as the example of there being no clear lines as to good and bad, but come one, Atticus, she is a horrible person and makes your kids feel like shit and calls you a nigger lover and you talk about how noble she is? What Lee writes in this new book about Atticus actually helps make sense of some troubling scenes I've been dealing with in a couple decades of teaching the book.
But I get it. Atticus is a great character and he is changed with this book.