I read this book when I was younger, definitely read it in the nineties but may have been published earlier than that. I remember several facets of the story, but can't recall the title.
The novel revolves around a family that was cursed such that one child in any set of twins born in the family would end up becoming a werewolf. I believe this happened around puberty.
There was also a male protagonist named Wolf (who was not a werewolf—I believe it was a Native American character). At one point in the story he meets a woman who puts him into some sort of trance/hallucination.
Finally, I remember the cover of the paperback novel. It had an animated picture of a man and a woman standing next to each other (you could just see their torsos). The man was holding a hunting knife. In the background there may have been a shadowed skull of some sort.
EDIT: There also may have been some World War I or II element to it- I remember that the story spanned quite a long time, and it took place, at least in part, around that time.