Oldboy

In 1988, a businessman named Oh Dae-su (Choi Min-sik) is arrested for drunkenness, missing his daughter's 4th birthday. After his friend Joo-hwan (Ji Dae-han) retrieves him from the police station, they go to a phone booth for Dae-su to call home. While Joo-hwan is talking to Dae-su's wife, Dae-su is kidnapped, and wakes up in a sealed hotel room where food is delivered through a trap-door. By watching the television, Dae-su learns that his wife has been murdered and that he is the prime suspect. Dae-su passes the time shadow-boxing, planning revenge, and attempting to dig a tunnel to escape.
In 2003, 15 years after he was imprisoned, and just before digging himself to freedom, Dae-su is hypnotized and wakes up on a roof-top with a suit. After interacting with a jumper on the rooftop, Dae-Su tests his fighting skills on a groups of young toughs and is mysteriously handed a cell phone and money by a beggar. He receives a taunting phone call from his captor, who refuses to explain the reason for his imprisonment. Later he collapses at a Sushi restaurant and is taken in by Mi-do (Kang Hye-jung), the restaurant's young chef. After he recovers, Dae-su tries to find his daughter and the location of his prison. He discovers that his daughter was adopted by a Swedish couple, and gives up trying to contact her. Dae-su locates the Chinese restaurant which made food for his prison, and finds the prison by following a delivery man. Apparently it is a private prison where people can pay to have others incarcerated. Dae-su enters the prison and tortures the warden, Mr Park, who doesn't know the identity of Dae-su's captor but reveals that Dae-su was imprisoned for "talking too much." While leaving the prison, Dae-su is attacked by a large number of guards but manages to defeat all of them.
Dae-su's captor, a wealthy man named Lee Woo-jin (Yoo Ji-tae), contacts Dae-su again and gives him the following ultimatum: if Dae-su discovers the motive for his imprisonment within 5 days, then Woo-jin will kill himself. Otherwise, Woo-jin will kill Mi-do. As Dae-su and Mi-do become intimate, they have sexual intercourse. Meanwhile, Joo-hwan tries to contact Dae-su with some important information about Woo-jin's sister, but is murdered by Woo-jin, who was secretly following him. Dae-su eventually recalls that he and Woo-jin had gone to the same high school, and that he had witnessed Woo-jin committing incest with his own sister. After Dae-su told his classmates about the event, Woo-jin's sister committed suicide, leading Woo-jin to hate Dae-su. Back in the present day, Woo-jin cuts off Mr Park's hand, fulfilling an earlier threat by Dae-su, causing Mr Park and his gang to (seemingly) join forces with Dae-su. Dae-su leaves Mi-do with Mr. Park and leaves to face Woo-jin.
At Woo-jin's pent-house, he is beaten badly by Woo-jin's personal guard, until Woo-jin intervenes, killing his own guard. Dae-su apologizes for driving Woo-jin's sister to suicide, upon which Woo-jin reveals the truth, that Mi-do is Dae-su's daughter. Woo-jin had arranged their meeting by using hypnosis to guide Dae-su into the Sushi restaurant, hoping that they would fall in love, so that Dae-su might experience the same pain which Woo-jin had. Woo-jin proceeds to threaten to tell the truth to Mi-do, who is being held by Mr. Park in his new prison. Dae-su begs him not to, and when Woo-jin is unswayed, Dae-su cuts out his own tongue as a symbol of penance. Woo-jin accepts Dae-su's pleading, puts down the phone and shoots himself in the head. In the aftermath of the event, Dae-su finds the hypnotist from the prison to erase his knowledge of Mi-do being his daughter, so that they can stay together and have a nice relationship. Mi-do then finds Dae-su and embraces him. Dae-su breaks into a wide smile, which is then slowly replaced by a look of pain, bringing into question whether the hypnosis worked.

Question: Whose sister committed suicide?
Answer: Woo-Jin's sister