Narina siblings

This book is about two siblings that are really close, they spend their entire life loving each other. A boy named William and a girl named Callie.They have and abusive dad so most of their times ends up being in the woods playing Narnia. They go through think and thin, and no longer have a mother due to a mystery. The siblings are twins, and get $50 every month from their grandparents without papa knowing. The kids raise themselves cause papa is just a bum and the kids know that. The kids never leave each other's side until...