Earthlings by Sayaka Murata

Natsuki isn't like the other girls. She has a wand and a transformation mirror. She might be a witch, or an alien from another planet. Together with her cousin, Yuu, Natsuki spends her summers in the wild mountains of Nagano, dreaming of other worlds. When a terrible sequence of events threatens to part the two children forever, they make a promise: survive, no matter what.