I'm going to go with her being 21, college student just shy of graduating. I figure with the age gap there's probably some generational disconnect going on with them (hard to have a lot in common since he would have been nearly grown by the time she was old enough to do more than just be an utter annoyance--unless he was a ten year old down with diaper duty? haha.).
I like the idea of her seeing something about their parents, maybe she's entirely too wigged out about it to admit what she saw. Could be she decides her brother is the only one she can confide in...or the one person she SO doesn't want to tell.
I'm on AIM at thegentlearcher if you want to chat about this a bit.