The three sisters in the movie are Yoon-hee, the oldest and most successful sister who works as a doctor in Seoul; Yoo-jin, the middle sister who is struggling to make ends meet as a single mother; and Mi-ok, the youngest sister who has a traumatic past and is emotionally distant from her siblings.
Their father's birthday party is supposed to be a happy occasion, but tensions rise when Jinseob, the troubled younger brother who has been struggling with mental health issues, shows up uninvited and causes chaos.
As the sisters try to deal with Jinseob's behavior, they also confront their own issues and conflicts from the past. Yoon-hee is still bitter about being forced to take care of her younger siblings when their mother abandoned them, Yoo-jin resents being the middle child and feeling ignored by her family, and Mi-ok struggles with the trauma of being sexually abused as a teenager.
Throughout the movie, the sisters slowly reveal their secrets and confront their unresolved feelings towards each other. As they try to navigate their family's dysfunction and cope with Jinseob's illness, they ultimately come to a greater understanding and acceptance of each other.
Three Sisters is a poignant and emotionally powerful exploration of family, trauma, and healing. It showcases the talents of the film's all-female cast and highlights the importance of communication, forgiveness, and empathy in overcoming the challenges of life.