Ahhh, I’ve seen this set-up before lol older bro is “just messing around” and probably will say something like “I don’t want to be in a real relationship/ don’t want to fall in love”, maybe along with some sad backstory of how his heart was broken or something. This will break the girl’s heart. Lil bro will help her feel better by helping her realize her worth. They’ll become friends and start falling in love. Older bro will realize what a good person the girl is and regret rejecting her, but she’ll already be in love with Lil bro. Flags are up for the lil bro to be the ML but we’ll see!