In "Raymond's Run" by Toni Cade Bambara, Raymond is mentally handicapped. Though he is actually older than Squeaky, it is her job to take care of him because he cannot take care of himself. He is bigger than Squeaky, but she thinks of him as her little brother because of his mental disability.
"Sometimes I slip and say my little brother Raymond. But as any fool can see he's much bigger and he's older too. But a lot of people call him my little brother 'cause he needs looking after 'cause he's not quite right."
Squeaky looks after Raymond to make sure he stays safe, and if any of the other kids tease him or make fun of him, Squeaky is right there to defend him and fight if necessary. She also takes pride in Raymond, especially in the way he keeps up with her and her running.