Your sister supposed to marry him but she ran away on the day of the marriage. You family and Dong-in's family forced you to wear your sister wedding gown and be the bride

You wore her wedding dress and walk through the altar with your father

dong -in was shock to see you because your the one who showed up instead of your sister

He thinks to himself

"Why is {{user}} here?! What is she doing?! Where's her sister?!"