You Again

The film opens in 2002 with Marni (Kristen Bell), an acne-riddled high school senior at Ridgefield High School, with glasses and braces, making a video about how much she hates high school, and reveals how she is tormented and bullied by other girls, specifically Joanna (Odette Yustman), who made Marni's high school life miserable. She adds that her protective older brother, Will, (James Wolk), was very popular as a handsome basketball player. However, at a very important basketball game, Joanna pushes the mascot, Marni, who runs into Will, resulting in a loss of the game.Years later, in 2010, however, Marni is a successful public relations executive, recently promoted to a job in New York. She has shed her "ugly duckling" look and is a fairly attractive young woman. When Marni flies home to her older brother's wedding, she discovers Will is about to marry Joanna, the very same cheerleader who bullied and tormented her throughout high school. When Marni meets Joanna for the first time in eight years, Joanna seems to not recognize her. Marni is also upset to see that Joanna fits in very well with the family. The plot thickens when Marni's mother Gail (Jamie Lee Curtis) meets up with Joanna's aunt Ramona (Sigourney Weaver), a woman who pushed Gail into a pool at their senior prom. Ramona is now a successful, wealthy woman who owns several hotels and a private plane.Although Gail seems willing to put the past behind her, she still feels the need to "outdo" Ramona during their interactions. On the other hand, Marni is unwilling to forget the things Joanna did to her in high school, unless she apologizes, and decides to try and let her brother know of Joanna's bullying past. Her attempts to get Will alone for a conversation fail. When Marni tells Joanna that she knows who she really is, it is obvious that Joanna does remember Marni. She refuses to give Marni a meaningful apology, and treats her disrespectfully, leaving Marni convinced that Joanna has not changed. Meanwhile, Gail comes to Ramona's hotel room for "closure" about anything bad between them that happened in the past. They appear to make up, but Ramona still seems not to be too fond of Gail.One day on the street, Will and Marni's flaky grandmother Bunny (Betty White) meets a man named Tim (Kyle Bornheimer). Joanna reveals that they have dated, and Tim appears devastated when he hears that she's marrying Will. Marni decides to bring Tim to the rehearsal dinner as part of her plan to stop the wedding. When it is time for guests to make a toast to the bride and groom, Tim unexpectedly jumps up to give his toast to Joanna. He reveals to a stunned wedding party that Joanna left him at the altar. Later, a video is presented (recovered by Marni) from their old high school timecapsule. The video reveals Joanna as she was in high school: an alpha-female bully, with the footage showing proof of her tormenting Marni. Will unplugs the video projector before the video is complete and walks out, furious.However, Marni is in trouble when everyone discovers that she was responsible for the video at the rehearsal dinner. Marni decides to talk with Joanna, hoping to reconcile, but Joanna is too furious and tells Marni "I thought we could start over." Marni is now convinced that Joanna hasn't changed, and she too walks away. Joanna then starts a fight with Marni, who fights back. Will walks in and witnesses the fight. He tells Joanna that he doesn't care about her past, but is angry that she was not being honest with him. He then tells Marni that he is furious at her for going behind his back to try to demolish Joanna's character. Both Marni and Joanna refuse to apologize to Will as well as each other for their actions.Meanwhile, Ramona and Gail argue after the rehearsal dinner, and Ramona accuses Gail of trying to upstage her as well as ruin her life throughout high school. A cat-fight ensues, with both of them falling into the pool. Ramona reveals that she always hated Gail, even when they were supposedly friends, because she always competed with and outdid her, culminating with Gail taking the guy that Ramona wanted to the senior prom. She sarcastically thanks Gail for what she did, because it motivated her to become successful in life. Gail apologizes for being insensitive, but reveals that she is proud of Ramona, and that her family is her accomplishment. Ramona, remorseful of her actions, tells Gail that she was jealous of Gail's family and feared that she was trying to take Joanna away from her. The two reconcile.Later that night, Marni finds Joanna in the kitchen binging on junk food. She finally admits to Marni that she feels truly awful for bullying and tormenting her and feels like an awful person, and that she loves Will. Marni forgives her and promises to get them back together. The next day, Marni pretends to be injured in order to attract Will's attention, and apologizes to him for her actions, saying she was only trying to protect him from Joanna.Joanna and Will reconcile in Marni's old tree house, but it collapses and injures both of them. They are both forced to stay at a hospital, which delays the wedding. However, Marni puts together a makeshift wedding at the hospital, with the bride and groom bandaged, but properly dressed and able to walk down the aisle. Marni appears to start a relationship with Charlie, her brother's good friend who was always kind to her.However, in a final twist-ending joke, Joanna introduces Marni's grandmother Bunny to her visiting grandmother Helen (Cloris Leachman). It is revealed in another montage of flashbacks that Helen and Bunny were enemies in high school when Helen stole a boy from her. However, Bunny gets her revenge when she cuts in on Helen's dance and takes her dance partner. Helen decides to seek revenge on Bunny... restarting another old freud.

Question: Why does Marni try to stop her brother's wedding?
Answer: His fiance bullied her in high school.