The Uninvited (2009)

After the death of her ill mother in a fire, teenager Anna tries to commit suicide and is sent to a mental institution for treatment. 10 months later, she still can't remember what happened the night her mother died, but her psychiatrist, Dr. Silberling, discharges her, telling her that she has resolved her issues. Her father Steven, a successful author, brings her back to their isolated mansion near the coast, where Anna finds that her mother's former nurse, Rachel Summers, is now her stepmother. Anna is happier to see her beloved sister Alex, who is swimming in the sea. Alex and Anna decide to look for evidence to prove that Rachel murdered their mother, as they investigate the fire in the boathouse.