Baby Sister
So not how the plot line goes. Yes, she is immature as has run away from prior commitments, and dropped out of school, and yes, there is an affair between her and her sister's boyfriend, but he makes the first move after he offers her a job at his medical clinic, and they have a traumatic night involving a patient who overdoses. With tensions and emotions running high, they kiss, which he initiates, and she accepts. But she is upset afterwards and doesn't want to continue as she loves her sister. It does end up continuing as he persuades her to stay working, as she at first advises she should find another job. However after joining him at a ballet event, which her sister couldn't attend, and asked her to go instead, they do finally give in to their emotions, and sleep with each other, as they both are in love with each other, but she is conflicted and decides to leave and return to school.
The affair comes to light when he is assaulted at his clinic, and she accompanies him to the hospital. When her sister arrives to see him she is informed his girlfriend was with him. It is questioned why she was still at the clinic and she tells her sister the truth. The boyfriend confirms it, and that he loves the younger sister, but says it no longer matters. Her sister and the boyfriend already had a troubled relationship. Annie doesn't want to hurt her sister, who is angry at the situation and feels betrayed, so removes herself from the situation, once her sister finds out and later leaves for the airport. Sister goes to airport to find her and forgives her as she is leaving.
Annie also has a troubled relationship with her father, as she believes he blames her for her mother's death when she was a child, as was playing in the street, and mother was killed by a car getting her. Also is resolved near the end, as he doesn't, and never knew that was what she thought and had been contributing to their distance and difficulties with each other.