Rascal Does Not Dream of Siscon Idol (light novel)

Sakuta had been waiting excitedly for a chance to spend time with Mai again, but instead he's greeted by a stranger who looks just like her. Apparently, Adolescence Syndrome has forced her to swap bodies with Nodoka Toyohama, a flashy up-andcoming idol...who also happens to be Mai's sister from a different mother! Now, forced to pretend to be the other, the sisters go about their respective lives as best they can but it seems like Nodoka is preoccupied with something besides walking in Mai's shoes...