Very Important. I need help to know the title or the Author

Here is what I know

Setting: New Zealand
Main Character Name: Soren
A girl who attends a private school takes her brothe to a store whereby the old man owner of the store puts a stamp on the little boy. The Stamp turns out to be the mark of an Incubus. The little boy is dying in the hospital and the girl must use the help of Soren, a male witch, to help save him.
I read this when I was a tween and I really want to find the book to own.
Thanks for your help.

Not a clue, personally, but there is an entire group dedicated to this kind of searches.

http://www.librarything.com/groups/namet...

Good luck.