I like my BFF big bro?!?!?!?! What should I do?

Me and my Best friend Mel are like sisters, I'm at her house about every other weekend and her parents the greatest. She's got a great big brother too, funny, sweet, adnormal and adorable. But the problem is that I've recently developed feelings for him. He has no idea (thank god) and neither does Mel. I tell her everything, but this, and I really want to tell her but idk how she'll feel, like she makes jokes all the time about me being a Potter (her last name) and that I should go to prom with him. Oh thats the other problem, He's almost 17 and I'm 14. What should i do. I really want to tell her. I like that me and him arent like sister and brother, we flirt and we fight like a couple but still. WHAT SHOULD I DO!!!!!