Please help? My bestfriend wants to date my older brother?

Ok, well my best friend wants to date my older bro who's 17 and her and I are 15. It's obvious they like eachother, even in school they are smiling at eachother! I want my bestfriend to be happy and be with who she wants but my older brother is just so ;...ew! like why would she wanna date someone who's so damn annoying? Maybe it's just cause I'm his sister, but EWWW! she's mad at me cause I won't be happy for her, what should I do?