Firmament: Gestern (Firmament #5)

Andi Lloyd is more comfortable than most with interstellar travel, but she's not prepared for the perils and peculiarities of a world she has all but forgotten—the planet Earth. As the Surveyor undergoes repairs, her brother August receives a message with news that will send both of them across the world to a place he never wanted to visit again.

Neither of them are prepared to be thrust into a world of political intrigue amid the tangled forests and crumbling ruins of Austria. They aren't prepared to encounter wild animals and endure cross-country hikes. And they definitely aren't prepared to face it all alone.

But despite the dangers they must press on into the unknown to find a way to save Andi's life, to decide the fate of Earth itself—and to rescue a lonely girl who just happens to be their little sister.