I'm looking for the name of a horror movie!?

In the movie, the main character is a teenage girl. At the beginning, she's taken out of the loony bin by her dad, an author, and taken home where her step mom is waiting, along with her sister, who you later realize died a year earlier and is a figment of the girls Imagination. Throughout the movie you are convinced the step mom is the bad guy, yet at the end you realize that the girl is truly crazy after she murders the step mom. She also accidentally killed her real mother, sick and in the guest house, and her sister. At the very end, she is taken back to the loony bin by her father, who is now left with no family except the crazy daughter.