Amber Altmyer is the second-born offspring of Bonnie Altmyer and her unnamed husband, younger than brother Harley (the film's main protagonist) and older than sisters Misty and Jody. The household was an abusive one, as all four children were subject some form of physical abuse from their father, while it was indicated (and later confirmed) that Misty was sexually abused by her father. It was also revealed that Amber harbored an incestuous infatuation with Harley, as she repeatedly fondled his private parts when they were kids.
The film's events had Harley as his sisters' legal guardian after Bonnie was arrested and convicted for killing her husband two years prior. Amber was established as being sexually promiscuous and verbally abusive towards Harley, making several threats to move out of the house. In one threat, she even attempted to take money ($1000) that Bonnie had saved up in her plan to leave her husband, planning to use it to get a license and leave, only for Harley to stop her after both of them took the money from Misty. Amber was in a relationship with a boy named Dylan, who was revealed as being abusive, as she was shown with a black eye later in the film. Despite this, Amber planned to move in with him, and informed Harley of her plans at his workplace in a grocery store. However, Amber asked Harley if he wanted her to stay, with Harley saying that he would until she finds something stable.
Amber backed out on moving in with Dylan, and it was shortly afterwards that her incestuous personality began to surface, as she was shown laying next to her brother wearing only a bikini and attempting to seduce Harley when he backed away in panic. She accused Harley of liking the affection, as he thought that she was Callie Mercer, Harley's married lover, after which Amber screamed at Harley over his relationship. Harley revealed Amber's incestuous past to his therapist, Betty, shortly after the incident, and later in the film, Amber turned heel and shot Callie to death in a jealous rage. Knowing that Callie and Harley's trysts take place in an abandoned shed, the evil Amber tracked Callie to that same shed and used a shotgun to kill her perceived romantic rival.
Harley found Callie's body after his confrontation with Misty--which followed the reveal that not only was Misty molested by her father, Misty turned heel herself and attempted to kill Bonnie out of jealousy, only to accidentally kill her father (Bonnie took the rap and ended up convicted). He was shocked and horrified to find Callie's body inside the shed, and after seeing Amber holding a shotgun, Harley realized that Amber had killed Callie. The deranged villainess shouted to Harley that she had no choice regarding her actions, while sobbing and wondering who would take care of her. Just as Bonnie did for Misty, Harley confessed to killing Callie to cover for Amber, and the film ends with the revelation that Amber and her sisters moved in with their uncle Mike.