Where was Joon at the beginning of the movie? Wasn't she supposed to be with Min-jung, her sister and her father? Why did Min-jung try to give away only Yu-jin to be saved?
I also wondered a lot about that so basically my theory is: Joon isn't Minjung's biological daughter. She took her in while they were still living with the army, possibly she is the daughter of one of the men, which would explain why she wasn't with the family at the beginning but they seem to have all the same history otherwise, and Yujin may even be too young to realize that Joon wasn't always with them. (Plus Yujin is the only one to actually mention her father directly, so, I hope that thought helps!)
