Second Child — John Saul

Beware the second child.

This lush, secluded Maine seaside resort is the summer playground of the superrich, but one hundred years ago, something disturbed their play. Horror came to this village. And though no one knows it yet, the horror has never left.
 
It waits for a shy young girl, outcast by her friends, her beautiful sister, even her own mother. She knows how it feels to be unwanted. She knows anger. And soon she will know the touch of unholy terror -- and the rage of blood-drenched vengeance.