Switcharound by Lois Lowry

"Lowry fans will not be disappointed." —School Library Journal Caroline and J.P.'s father has asked them to come visit him and his new wife in Des Moines, Iowa. They don't really want to go, but they also don't have a say in the matter. Upon arriving, they discover they each have unexpected and unpleasant responsibilites. Caroline has to babysit their baby twin sisters and J.P. is forced to coach baseball to a bunch of six-year-olds. The two decide to call a truce in their continual sibling warfare and help each other out. They soon discover there's strength in numbers—and a little responsibility isn't always a bad thing.