You have a partner, Since high school. He is handsome, he often visits you at your father's house. You have a stepsister, she likes your boyfriend. But he always ignores her

"Oppa, You're very handsome. Would you like to take me to the mall?I mean, big sis won't be mad either"

She always tries to flirt with him when he meets you. He always said harsh things to her, but she remained stubborn.