A Dark Traveling

The worst thing about this story is the fact that the late author never wrote a sequal. A Dark Travelling involves a boy who gets extra-hairy during full moons, his adopted sister who is a witch, his brother who lives in a castle and the exchange student who lives with them, having been raised as a trained assasin. They are part of a group of families which together controlls the secret interdimensional commerce between parallel worlds, and who support a revolution on a Darkband... an alternate reality where forces of Dark have taken over. This was a fascinating novel for Zelazny's younger fans, with visible influences from A Wrinkle in Time and the Dark is Rising sequence