Kind Younger Sister (2018)

Jung Geun, who was dating Seon Hye, his first love in high school, separated due to his enlistment in the military and Seon Hye's study abroad. His father suddenly announces remarriage and he has a meeting with his stepmom's family. Isn't his stepmother's daughter Seon-hye?! Jeong-geun and Seon-hye are shocked. However, their past could not prevent their parents from being happy, so their parents eventually remarried, and the two became brothers and sisters and lived in the same house.