Reading progress update: I've read 138 out of 353 pages.

Reality Boy - A.S. King

This family dynamic is very similar to the one in Ender's Game.  Evil oldest child, angelic middle child, youngest boy who the world revolves around is simultaneously never left alone and completely ignored.

 

It also makes me think of Perks of Being a Wallflower.  However, this boy actually thinks and reacts like a teenager with problems rather than being overly sophisticated (and utterly naive at the same time) for his age like the boy in that book was.