Suzunari! 

Kaede Takamura's teenage life swerves to the brink of insanity when she comes face to face with her twin...or does she? Suzu is practically the mirror-image of Kaede - except for having cat ears and a tail and being far more well-endowed than Kaede thinks any teenager ought to be. But the Takamuras have no qualms about bringing this strange, happy-go-lucky catgirl into their home. With Suzu completely ignorant of the ways of modern society, it's going to be a very long year for poor Kaede.