it is probably a bit of jealousy, just low-level jealousy that falls under human nature. you've never been irrationally jealous over something you only kinda want before? should be easy to keep in check though. your situation sounds kinda awful (and a bit north of fuckbuddy to be honest), and I don't see more than two outcomes.
you could take it to the next level and she would be happy for a while, but would you? you really don't seem that enthused with her...this also disregards the fact she is throwing up all sorts of red flags. it is worth noting that she is probably in a situation she does not want to be in and that is probably exacerbating the crazy, but who knows! there is also the issue with your passive-agressive bro, you
stealing her would probably make things wierd for a bit. then again, this could be remedied by telling him not to worry, there is a 98% chance he will get some of that rebound action when it doesn't work out!
other option is to start putting some distance between you two. if she is really your friend, it would be shitty to straight-up bail on her. hook up less. this may be taken as a straight-up bail by her though, and you may lose your friendship. which happens a lot when you sleep with your friends. just try and keep communication open, be honest within reason (tact!), and hope for the best.
if it goes wrong, hopefully you haven't told her how much your strange friend's behavior bothers you, cause if she's pissed, there is a chance she'll fuck him to piss you off. this doubly sucks because he will then think there are nothing wrong with his methods, and keep on creepin on. there is nothing worse than when lame behavior is positively enforced.
