Trying to remember the name of a book I read in 6th grade. A teenage boy and older teenage girl (siblings) lose their parents. They're being hunted by an older man. I remember their place being robbed, and I remember that the girl works on the beach to make money. I think the ending had something to do with a picture on the wall? And I think the title had something to do with Invisible or Stranger. Been searching Google for an hour. Please help!