Y’all I don’t think she has actually let the idea that her sister did on purpose sink in. She is just “selfishly” acting that way because she does not want to believe in it. Chulsoo might have made the connection but she does not want to admit it to herself, since it would mean that she almost killed everyone in the orphanage and took her place. It’s just human nature, it does not make her a bad person. She finally found her sister again and she wants to keep her close