Earthlings by Sayaka Murata

Natsuki isn't like the other girls. As youths, she and her cousin Yuu spent the summers in the wild Nagano mountains, hoping for a spaceship to transport her home. When a terrible sequence of events threatens to part the cousins for ever, they make a promise: survive, no matter what.

Now, Natsuki is grown. She lives quietly in an asexual marriage, pretending to be normal, and hiding the horrors of her childhood from her family and friends. But dark shadows from Natsuki's past are pursuing her. Fleeing the suburbs for the mountains, Natsuki prepares for a reunion with Yuu. Will he still remember their promise? And will he help her keep it? Dark, sharp and with a deeply unexpected twist, Earthlings is an exhilarating cosmic flight that will leave you reeling.