Yay hi

Hey, I’m new here. I am looking for a story that I read a long time ago, and I am not sure where I read it. It was about a boy who was regressed by his aunt (I think) and moved in with his aunt’s kid (can’t remember if it’s a boy or a girl). Eventually, he gets used to being treated like a baby and starts to act like one. The story is really old so this is the best summary I can give. Any help would be appreciated.