Nurse 3-D

The film opens with Abby Russell at a night club where she explains to the audience via a narration that her "job" is to murder cheating men in order to "keep scum off of the streets"- a job that she thoroughly enjoys. She then murders a man by slicing through his femoral artery and then throwing him off the roof of the club. The following day she attends a ceremony where she watches Danni Rodgers, a nurse she had been mentoring, graduate. She then approaches her and meets Danni's mother and stepfather Larry Cook. Despite this joyful moment and all of her preparation, Danni is ill prepared for her first true day of nursing duty and is berated by Dr. Morris for her inability to respond quickly to an emergency situation. Abby views the situation with disgust, as she knows that Dr. Morris is a sadist that enjoys harassing new nurses.
Abby is further irritated when Danni chooses to call her paramedic boyfriend Steve for support rather than approaching Abby herself. She's pleased when Danni's call to Steve ends badly due to a fight the couple had had previously that night over Danni's refusal to move in with him. She explains her actions to Abby by saying that she is unwilling to trust her stepfather enough to leave him alone with her mother. This distrust is later justified when Abby and Danni witness her stepfather having an affair while the two women are on their way to go drinking at a nightclub. Danni is upset at the encounter, which makes her more willing to drink later that night. However unbeknownst to her Abby spikes her drink with drugs, which enables Abby to get Danni to have sex with both her and random strangers. The next day Danni wakes to find herself in Abby's apartment and leaves despite Abby's entreaties for Danni to skip work and spend the day with her. After she leaves, Abby downloads several photos that she had taken from the previous night before leaving to see a psychiatrist, which is revealed to be Larry.
Abby seduces Larry by saying that she's addicted to men, alluding to her past history with her father. He gives her his phone number to contact him anytime she wishes, which seems to confirm Abby's suspicions that Larry is a cheater. She later approaches him while he's leaving his office and convinces him to give her a ride, which enables her to murder him by paralyzing him in his car with vecuronium bromide and putting it in reverse, which causes him to get into a car accident. After hearing of her stepfather's death Danni seeks solace from Abby, only for Abby to grow angry when Danni says that she's going to move in with her boyfriend. Abby comments that she hopes that Larry's genitals were cut off in the car crash, only for Danni to reveal that she had never told Abby how her stepfather had died and quickly leave. This infuriates Abby, who decides that instead of seeking to help Danni, she will now hurt her. She accomplishes this by slowly convincing Detective John Rogan that Danni is mentally unstable and obsessed with Abby.
The next day, Abby runs into Rachel Adams, a new human resources employee that remarks that Abby greatly resembles a girl she knew that was sent to a mental institution. Abby invites Rachel out for drinks and takes the opportunity to harass Danni by calling her via Skype and showing Danni a video of Abby injecting chemicals into Rachel. Danni tries to go to the police, only for Detective Rogan to dismiss her claims as evidence of her trying to hurt Abby because the other woman didn't return her affections. He uses the photographs Abby took as proof to this effect, which Steve sees as a result of Danni calling him to the police station for support. This prompts an argument between the two and Steve leaves in a fit of anger. Danni tries to approach Dr. Morris for help, only for him to use this as an opportunity to blackmail Abby into having sex with him. Abby initially pretends to agree to this arrangement but soon shows that it was only a way for her to get him alone so she could dismember and murder him. That same night Abby also knocks out Rachel and drags her away to her death.
Danni goes to the mental institution that Rachel had referred to, seeking answers about what is going on. There, she learns about Sarah Price, a little girl who killed her father after witnessing him not only having an affair but also severely beating her mother for coming to his office unannounced. Danni also discovers that Sarah was taken in by one of the nurses at the institution, a woman named Abigail Russell. Danni then realizes that Abby is Sarah and has since taken the name of her caretaker. Danni then tries to call Rachel to warn her about Abby, only to find that Rachel's phone is in her car. She then receives a call from Abby, who implies that she will kill Steve in the same manner as Rachel.
Danni rushes to the hospital, where she and Abby begin to fight. The staff initially tries to intervene, only for Abby to set off on a killing spree and lock herself into a lab. Danni and Steve finally manage to get inside, only for Abby (who had hidden in one of the patients' beds) to stab Steve in the neck and run off. Abby rushes home to grab some essentials but is confronted by Detective Rogan, who tries to arrest her. Noticing that her neighbor Jared is viewing the situation, Abby pretends that Rogan is trying to rob her. Her neighbor then comes outside and hits the detective over the head with a bat, killing him instantly. Her neighbor is horrified to discover that Rogan is a cop, but Abby convinces him to hide the body, saying that Rogan was corrupt and that Jared would be treated badly since he is now a cop killer.
Abby, having fled, is shown to have assumed a new identity, that of human resources employee Rachel Owen.

Question: Who is having the affair?
Answer: Stepfather