In flashbacks, Sun is a little girl and breaks a glass ballerina, then lies to her father about it. As an adult we see her in bed with Jae Lee, saying she's sorry but she can't because she is married. Her father catches her in bed with Jae and she is mortified.
Later, when Jin comes home Sun seems nervous. They fight and he leaves to deliver a message for Sun's father.
Later, Sun attends Jae's funeral.