The Summer of the Swans, by Betsy Byars

Sara has trouble dealing with her older sister, her aunt, her absent father, and most of all, her retarded brother, whose needs always seem to outweigh her own. But then one night, her brother disappears and she must hunt for him.

It's a Newberry Award book, but it has aged badly. And beyond the references to a life long passed (operator assisted calls, anyone?), the whole psychology of 14 year olds has changed tremendously since this story was written. It's a pretty little book, but terribly trite and boring.