Voices

Kim Ga-in is a student in South Korea. She and her family come to her aunt Jee-sun's wedding but before the wedding, Jee-sun is pushed off the balcony and rushed to the hospital. Ga-in waits with her boyfriend, Park Hyun-joong, while her aunt recovers. They then witness Jee-sun's younger sister, Jung-sun, repeatedly stabbing her.
Jung-sun is arrested for Jee-sun's murder, and held for questioning. It is revealed that the family believes they are cursed and at least one member dies in incomprehensible ways. In this case, Jung-sun was possessed and killed Jee-sun. At school, Jung Eun-kyung, the top student, attempts to kill Ga-in with scissors. During the struggle, Eun-kyung is stabbed instead and transfers to another school. That same day, Ga-in is confronted by her teacher, who blames her for Eun-kyung leaving and tries to kill her. Ga-in is saved by a classmate.
Hong Seok-min, an eccentric student rumored to have killed his own father, advises Ga-in to trust no one, including herself. She visits Jung-sun to ask her about the murder. She tells her that Jee-sun's husband is Jung-sun's ex-boyfriend, and she had required revenge on Jee-sun. She says that an unknown force possessed her and coerced her to kill. That evening, the classmate that had saved her comes to have his try at killing her. She runs inside only to find her mother throwing knives at her. Frightened and no longer feeling safe inside her own home, Ga-in tells her father she is leaving. Her father tells her that there is a forgotten family member in a different village, named Hwang Dae-yong. On the bus, Ga-in meets Seok-min again, and together they visit Dae-yong.
Dae-yong tells them how he killed his wife in a fit of jealousy when he learned of his wife's alleged affair, then tried to find the force that controlled him after being released from jail. After Ga-in and Seok-min return home, Dae-yong commits suicide to stay away from the curse. At school, Ga-in has an apparition of her best friend killing her and realizes that she really cannot trust anyone. One night she wakes up and finds her parents killed. It is divulged that Hyun-joong is the one who killed her parents. Hyun-joong stabs her as she tries to save her younger sister Ga-yeon, and wants to set the house on fire to turn it into hell. Both sisters stab him and leave him dead as the house begins to burn.
In the hospital, Ga-in and Ga-yeon are placed in the same room. Ga-in is having nightmares and is woken by her sister. Ga-in has a hallucination of Ga-yeon taking a knife from her pocket; they struggle and the knife ends up stabbing Ga-yeon. Ga-yeon cries and asks Ga-in how she could do this to her own sister. Seok-min appears and exposes his true individuality: the curse. He can only be seen by Ga-in and the people he controls. Ga-in looks down at Ga-yeon and realizes that it wasn't a knife in her pocket, but instead a burnt family photo. Seok-min then tries to kill Ga-in. Ga-in stabs Seok-min, but in reality she stabs herself because he is controlling her from the inside.
In the epilogue, a boy is being verbally assaulted by his teacher. After the teacher leaves, Hong Seok-min appears in a new body and asks the boy if he wants help to get back at the teacher.

Question: What is the relationship between Ga-in and Ga-yeon?
Answer: Sisters