I read this book about 10 years ago. I believe the tile has the word river or water in it. I have looked through all the lists I can find and read the descriptions to find this book. There is a boy/young teenager taken to another world or universe where 3 siblings are fighting for control. The oldest brother is a vampire of some kind, the sister can transform into a dragon and the youngest brother I can't remember. They each want to use the boy and maybe a ring for their own uses. It man have something to do with dreams or time. It was dark, strange and complicated. I don't think it was for children and may have been a triology. I think the book begins as the boy and his family
I found it... it's called the Riverrun Trilogy - S. P. Somtow. Here is part of the amazon review:
It is WEIRD. It is a total trip. The family is separated and they are trying to save the others and they are dead and they are floating and they are going to save the universe and it is just WEIRD! If you are in the mood for something quite odd and floaty and don't mind some slow and a bit incongruous writing, go for it.
....and the author seems interesting as well Thai-American musical composer, writing ballets and operas. He is also a science fiction, fantasy, and horror author writing in English. Somtow has both Thai and American citizenship.