Rules by Cynthia Lord.
A well deserved Newbery Honor book, this is a lovely story of a girl dealing with the issues of being the older, responsible sister for an autistic brother. Her best friend is gone for the summer, and she is desperately hoping the new girl next door will be her pal, but is convinced her brother will ruin everything for her. Then she makes a surprising new friend who makes her question her own abilities to be a good friend, sister and person. Highly recommend for grades 5-7.