The Summer of the Swans

Newberry award book by Betsy Byars, published 1970.  Says it's for ages 8-12.  Story told by 14 year old girl, Sara who has a younger brother, Charlie.  Charlie had a serious illness when he was 3 and it damaged his brain.  He doesn't speak, and is behind developmentally.  They and an older sister live with an Aunt.  Their Dad works in another town and and comes home occasionally.

In the spring, swans come to a small lake nearby for a short time each year.  Sara and Charlie walk down to the lake to watch them and Charlie is fascinated by them.  That night Charlie disappears.  Touching story about the entire town trying to find Charlie, and Sara's emotions as she realizes how much she loves her brother, and finally accepts help from a boy she previously  thinks is a jerk who helps her find Charlie.  Touching story.  Probably good for teens maybe up to 16.