I believe you are talking about the movie "The Uninvited".
The main character Anna:
In it, the main character, Anna, is in a mental institution after a suicide attempt, following the death of her mother in a fire. When she returns home, she reunites with her sister Alex and her father, and some strange deaths occur, which the girls are convinced are related to their father's new girlfriend, Rachel.In one of the deaths, the body is pulled from the water (Their home is located on the New England coast.)
Alex actually died in the fire that killed her mother, and it is Anna that has been committing all the murders, and she realizes it just when they come to arrest her and take her back to the institution.