The Girl Next Door (2007)

In 1958, a pair of recently orphaned girls are left in the care of Ruth Chandler, a woman slowly going mad. Ruth decides that she must discipline the girls, and encourages her three young sons to share in the girls' punishments. David, their 12-year-old neighbor, has developed an innocent crush on the older sister. As the summer progresses, he finds himself a witness to her sadistic torture. He wants to stop the abuse, but first he must find an adult who will believe his unlikely story. More importantly, he must find the strength to betray his best friends and their mother--an adult he has looked up to all his life.