Shattered glass
As the twins' life starts to become increasingly more shattered, imagery of broken glass (especially glass mirrors) become more common. Mirrors reveal those who look into them on a deeper level. They reflect not just their physical appearance, but their soul. The shattered mirrors throughout the novel, some of which the twins smash themselves, show how fractured the twins are and Kaeleigh's personality is.
The carousel
The carousel, which appears frequently in the novel, represents the twins' childhood and, initially, their innocence. However, over time, as the carousel becomes more dilapidated and tarnished over time, their innocence wanes, too.
Drugs
As the twins, as well as the twin's mother, descend deeper into darkness and self-harm because of their increasingly problematic addictions to drugs and alcohol, imagery surrounding those drugs and alcohol becomes more common. This is also reflective of their inner state: they don't like themselves.
Sexual assault
Sexual assault imagery, which occurs when Kaeleigh is raped by her father, is often graphic and unrelenting throughout the novel. However, this imagery reflects the twins' father's perverse nature and the reason why Kaeleigh develops dissociative identity disorder as a result of the trauma her father inflicts on her.