Teasing You with a Love Triangle in a Quadrilateral

I like my older sister. Lu Wu likes her too, but she likes Lu Wu's older brother, and for now, he doesn't seem interested in dating. The upshot is that I absolutely can't let Lu Wu get near my sister.

"Heh, Lu Wu, my sister rejected you again, didn't she? I told you she'd never like you."

"Yeah, so I don't like your sister anymore, Lin Huan! I like you!"

"Get lost! I'm a man, damn it. Why do you like me?"

"I'm so used to you bullying me that I fell for you before I knew it."

P.S. This story has a gender-swap marriage plot. (Obviously.)