Who to Trust? Trust Yourself.
You made some decisions with EXCELLENT reasons, I might add. You were absolutely right: this girl was trying to play two ends against the middle and got burnt.
How the fvck does she have your friends' phone numbers, anyway? You need to talk to your friends and tell them your story in a very short, direct, and calm way. That story is that this chick had been traveling four hours to see a guy she had the hots for, invited him to stay the weekend with her, and then spent the weekend with him. When you had a problem with that, she told you that she "goes from relationship to relationship" and that she "needs her space" all a huge sign of disrespect to you, regardless of whether she actually fvcked this guy or not (and, hey, I think we know the true answer to that ... that would be an affirmative).
And then tell your friends not to be such fvcking whimps listening to her, that they're going to have to decide ... they can't be her "friend" and allow themselves to get in the middle AND be your friend. She is using them.
FORTUNATELY, THOUGH, YOU'LL KNOW WHO YOUR REAL FRIENDS ARE. The ones who are disloyal to you and continue contact with her after you've been plain with them are not your friends. They are spineless disloyal ****roaches.
Then, you simply tell them that you will have NOTHING TO DO with talking to them about her. You will render them powerless. They cannot be part of the game, they cannot be used, if you do not allow them to speak to you about her. If they persist and talking to you about her and conveying messages, you simply tell them that you are firm, bid them good day, and hang up. Don't take calls from friends who are disloyal. You might have to write some people off here bro.
Importantly, you cannot bring up this girl to these people either. You can't forbid them from getting involved and then talk to them about her, either. Get yourself a therapist (or a dog) to talk to if you really need to talk about it. Or, post more often here. We'd be happy to talk about it here with you.
I feel for you brother, it must be tough. It's a tough situation and we can feel your pain. But, you have seen the light as clear as day. She has shown her true colors. You must stay away from her brother. She has blown the trust. It cannot be reinstituted. There is no "reset" button on trust. Once it is gone, it is gone.
Even if you were somehow to allow yourself to get back with her, would you ever fully trust her now? You'd always sleep with one eye metaphorically open after this, AND WITH GOOD REASON. She was willing to disrespect you, to be disloyal to you, and fvck with your relationship, your emotions, your wishes, and you as a friend and lover FOR WHAT???? She TOLD you that she likes to go from relationship to relationship --- WHAT THE FVCK????
She was testing the waters with this guy, THINKING foolishly that little chumpboy Jet would still be around to take her after she got through swallowing some other guy's load and that YOU, JET, being a weak puzzyboy, would just take her back.
W-R-O-N-G. You're no chump, Jet. You're no puzzy. I'm afraid she's misplayed her cards and, of course, NOW SHE'S FULL OF REGRET. AND, TO BOOT, it didn't work out with the studman. Hahaha. Fvck her.
Go get a woman who is deserving of you. This one clearly is not. And, find out who your true friends are and who the pretenders and AFC puzzies are.
Consider yourself blessed. This is an excellent opportunity. It might have taken you years in an otherwise uneventful life to separate the wheat from the chaff and find out who your "true" friends are when the chips are down, and to find out the true colors of this girl, AND to test your own metal and resolve.
Good luck and keep us posted.
