The Girl Next Door by Jack Ketchum

Suburbia. Shady, tree-lined streets, well-tended lawns and cozy homes. A nice, quiet place to grow up. Unless you are teenage Meg or her crippled sister, Susan. On a dead-end street, in the dark, damp basement of the Chandler house, Meg and Susan are left captive to the savage whims and rages of a distant aunt who is rapidly descending into madness. It is a madness that infects all three of her sons and finally the entire neighborhood. Only one troubled boy stands hesitantly between Meg and Susan and their cruel, torturous deaths. A boy with a very adult decision to make.