Kym Buchman (Anne Hathaway) is released from drug rehab for a few days so she can go home to attend the wedding of her sister Rachel (Rosemarie DeWitt). At home, the atmosphere is strained between Kym and her family members as they struggle to reconcile themselves with her past and present. Kym's father Paul (Bill Irwin) shows intense concern for her well-being and whereabouts, which Kym interprets as mistrust. Kym also resents her sister's choice of her best friend to be her maid of honor instead of her. Rachel, for her part, resents the attention her sister's drug addiction is drawing away from her wedding, a resentment that comes to a head at the rehearsal dinner, where Kym, amid toasts from friends and family, takes the microphone to offer an apology for her past actions, as part of her twelve-step program.
Underlying the family's dynamic is a tragedy that occurred years previously, which Kym retells at a Narcotics Anonymous meeting. As a teenager, Kym was responsible for the death of her younger brother Ethan, who was left in her care one day; driving home from a nearby park, an intoxicated Kym had lost control of the car, driving over a bridge and into a lake, where her brother drowned. The day before the wedding, as Rachel, Kym, and the other bridesmaids are getting their hair done, Kym is approached by a man whom she knew from an earlier stint in rehab. He thanks her for the strength she gave him through a story about having been molested by an uncle and having cared for her sister, who was anorexic. Rachel, hearing this, storms out of the hair salon. The story turns out to be a lie, an apparent attempt by Kym to evade responsibility for her addiction.
The tension between the sisters comes to a head later that night at their father's house, when Kym comes home. Rachel reveals she has never forgiven Kym for their brother's death, and suggests that Kym's rehab has been a hoax since she has been lying about the cause of her problems. Kym finally admits responsibility for Ethan's death and reveals that she had been relapsing in order to cope. She gets into her father's car and leaves. Kym heads to the home of their mother Abby (Debra Winger), hoping to find solace with her. However, a fight breaks out between them, when Kym asks Abby why she left Ethan in her care on the night of his death despite knowing that she was often on drugs, suggesting that Ethan would have been better off in Rachel's care. Abby tells Kym she left Ethan with her because she was good to him and that she thinks Rachel is a hypocrite for her accusations.
When Kym makes it clear she thinks her mother's decision was in part responsible for Ethan's death, Abby becomes furious and punches Kym in the face. Kym hits her mother back and drives off in her father's car. While driving away, Kym begins sobbing uncontrollably because she feels Abby has not accepted appropriate responsibility for her part in the actions which ultimately caused Ethan's death. Kym drives the car off the road in an attempted suicide and crashes into a boulder. Rather than summon help, she spends the night in the car while everyone at home worries about what has become of her. The next morning, the day of the wedding, Kym is spotted in the car by passing joggers, who call the police. The police awaken her and give her a sobriety test, which she passes. She gets a ride home with the driver of the tow truck who is towing the wrecked car. She makes her way to Rachel's room, as Rachel prepares for the wedding.
Seeing Kym's bruised face from the crash prompts her anger of the previous night to vanish, and Rachel tenderly bathes and dresses her sister. Amid a festive Indian theme, Rachel and her fiancĂŠ are wed. Kym is the maid of honor, and is overcome with emotion as the couple exchanges their vows. Kym tries to enjoy herself throughout the wedding reception but continues to feel out of place and is plagued by the dispute with her mother. Ultimately, her mother leaves the party early, despite Rachel's effort to bring the two together, and the feud between Kym and Abby is left unresolved, suggesting Abby's emotional distance and unwillingness to accept responsibility is the root cause of the family's problems. The next morning, Kym returns to rehab. As she is leaving, Rachel runs out of the house to hug her.

Question: Why does Kym's mother punch Kym in the face?
Answer: because Kym blamed her mother in part for her brother's death
Alternative answer: Kym told her she is partially to blame for Ethan's death.