Anna is the killer.
She accidentally killed her mother and her sister Alex when she tried to set her house on fire with her Dad and Rachel in it. Alex is only in Anna's mind.
Anna also kills Rachel. Mildred Kemp is actually one of the other patients at the mental ward where Anna was at for almost a year. Anna also killed Matt when she realized that he saw her accidentally kill Alex and her mom. She gets sent back to the mental ward where she tells her Doctor that she did what he told her to.
She finished what she started and killed Rachel.
The movie ends with Anna smiling at the camera