Many Waters by Madeleine L'Engle

Sandy and his twin brother Dennys, are the practical, down-to-earth members ofthe Murry family. They have never paid much attention to their scientist parents' talk of highly theoretical things like tesseracts and farondalae. But now something has happened to Sandy and Dennys that drastically stretches their powers of belief. And, when disaster threatens the oasis where they have made their home, can they find a way back to their own time?