I read this a long time ago, probably about 10-12 years. The entire story was in universe (no going from normal earth to some other universe), and focused on two brothers and possibly a sister. One of the brothers gets turned into wolf (or at least a wolf / human hybrid) by the villain. I think it may have been part of a series as I don't seem to remember the book ending conclusively. Also I think the cover had a tree on it, maybe.
Sorry I can't remember more, it has been a long time ...
