The Stepdaughter (2000)

Jesse Conner runs a ranch in California with his ailing wife Maggie, his daughter Linda, and his son Buddy, who goes to a special-education school. When Will, one of their horse handlers, is murdered at a hotel in Los Angeles, Jesse hires Susan Heller to fill Will's position. Linda, who has a large collection of mystery novels, is immediately suspicious of Susan, so she decides to get Jesse's mechanic Paris, to help her figure out whether or not Susan is up to something, but he tells her that she's been reading too many mystery novels and needs to lighten up. Maggie has an accident in which she falls off of a ladder and is taken to a hospital where she is treated, and as she's released, Nurse Callista Davis recognizes Susan and calls her Karen, which arouses Linda's suspicions even more. Jesse later explains to Susan that he once bought Maggie a horse named Big Palomino, and Maggie fell off of the horse one day and needed hip replacement surgery and has had to take pain medication ever since. Linda calls Callista, who comes over to the ranch while no one is around, and Susan uses a big knife to kill Callista, then leaves Callista's body in a lake. Linda, who is not aware of Callista's murder, asks local deputy Tom Gitlin to check Susan out, but he finds nothing. On Susan's birthday, while everyone is away, Susan confronts Maggie alone while Maggie is taking a bath, and tells her that she is her daughter; her real name is Karen Mills. When Karen was born, Maggie's parents forced her to give Karen up for adoption. For her entire life, Karen has been hurt by the belief that she was never wanted. She endured an existence in institutions and foster homes, and now she wants revenge on Maggie for giving her up to that kind of life. She finishes by dropping a radio into the bathtub water, which electrocutes Maggie to death. Linda figures out that Callista once worked as a night nurse at the Carson Correctional Facility in Los Angeles, and Karen was one of Callista's patients. Linda later steals a file from the human-resources cabinet, and figures out that Karen was at Carson for a few years because she burned down a house, killing a foster father who was abusing her, but Linda is unaware that Karen killed Will so Karen could take over his position at the ranch to get close to Maggie. At the ranch, Karen insults Buddy, who is on the phone with Linda, and Buddy runs away from Karen. Karen heads to a nearby cave that Buddy likes to hang out in, and Paris, who wants to sleep with Karen, confronts her. Karen sees Buddy run out of the cave, and Karen sends Paris falling to his death, down a drop-off in the cave. Karen leaves the cave to find Buddy. A few minutes later, Linda arrives at the ranch to help Buddy try to fight Karen off, but Karen might be too much for them to handle.