The two twin sisters were the slaves of the doll Mary-Ellen, they listened to every word she said, Jillian thought it was all in their imagination, and she found it annoying that the doll was all they talked about. Later in the story, Mary-Ellen brings the evil dummy Slappy to life so that Slappy will be her new husband, but Slappy didn't want her, he wanted Jillian. Slappy and Mary-Ellen fight, then he pushes her into a saw, and she pulls him in with her. Now Katie and Amanda are finally free from their slavery.
The two girls think alike and do everything together, they love their sister Jillian, but in a way, they kind of listen to everyone, but then again, Mary-Ellen threatened them.