I'm not usually one to be shocked by a movie but that film was very uncomfortable to watch. It wasn't so much the aunt or her actions but more the action/inaction of the other children that was really chilling.
The Girl Next Door - 2007 (Not the one with Elisha Cuthbert in, but Jack Ketchum's film about teen/family abuse).
Very very shocking movie, and very upsetting at the same time, it delivers a powerful message to the viewer about brutal and sexual abuse. Although I thought the film was quite hard hitting, it was made really well, it certainly gets the message across and quite disturbing to watch.
http://www.imdb.com/title/tt0830558/
I would give this film a 8/10