Many Waters (Time Quintet #4) by Madeleine L'Engle
Some things have to be believed to be seen.
Sandy and his twin brother, Dennys, are the practical, down-to-earth members of the 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?