oh wow, thats the pittiest funniest irony ever!! that's really very sad but i'd really like to put my suggestion here too! If you and that girl are totally over, i mean if you guys arent FBIs anymore, and don't share any physical relationships then i dont think it'd be a problem unless she's a big time whore! but i also dont think it's the best decision to hide it from your own brother because your brother's going to marry her! you wouldnt want your brother to marry someone you've had sex with! so I guess you should tell your brother or talk about it to the girl, what she thinks about it and deal with it! :)