I’m sorry if you took it as a critique-I posted that so that others would know that it was already reported, eliminating multiple posts to the Mods.
I’d keep quiet. Because you don’t know what really goes on inside of other people’s relationships. It’s possible that your brother suspects but doesn’t WANT to know, for whatever reason. Maybe he’s happy the way things are and doesn’t much care. Maybe they are companionable but have differently calibrated sex drives. Maybe he’s also cheating on her but doesn’t want to break up just yet. You never just never know.
And the other danger of telling all is that’s its very possible that they will kiss and make-up and he will forgive her. And still remember all the bad stuff you said about her, his most truest love that can do no wrong. And she will hate you for trying to break them up. This is why I never criticize my friends partner’s while they are fighting with each other. And why I don’t get involved in their fights. Because the lover will forgive all the bad stuff in a way the lovers best friend won’t.
I never get this “keep out of it” attitude. If your brother had a low tire, you’d tell him, right? You wouldn’t be afraid that he secretly suspected but never wanted to find out, so now he’s all upset? Or you can’t tell him if his fly was open because people have different fashion sense, and there’s a risk he’d never speak to you again?
But something affecting a family member or a close friend that can have real repercussions, you think MYOB is the only solution? So when he eventually finds out, and finds out that EVERYBODY, including random strangers on the internet, knew before he did, that won’t be a problem? That he won’t hate your for NOT telling him?
OK.
There is a saying in the Biker World:
Bros Before Hoes
You’re such a rake.
ISTM the decider is how close the relationship is between teller and tellee.
Close family? Of course you tell what you know or think you know. Being very explicit about the uncertainty of your info.
Some random cow-orker you pass occasionally in the halls? Your intel better be damn solid before you stick your head in there. And even then you’re choosing to deputize yourself when another hundred cow-orkers are as well- or better-positioned than you are to know the facts and spill the beans. Remember the WWI motto: Heroes get shot.
IMO the advice gets dumb when somebody claims to have a one-size fits all policy.
All in all, crappy people create crappy situations. In those situations even righteous people nearby get splattered with crap. Some folks value not getting splattered more than they value saving a friend or family member. ISTM those folks are a different kind of crappy.
I won’t associate with a friend that is cheating with a best friend or any other kind of blatant betrayal. Cheating is bad enough but betrayal of a friendship brings it to a completely different level. I would not hesitate to snitch her and him out and end the friendship immediately. She should have broken up with your brother if she is going to cheat.
Hey Wendy92: When you decide to tell your bro about his cheating GF, be sure and come back to tell us about how it went here: http://boards.straightdope.com/sdmb/showthread.php?t=843018 
Push Me, Shovel You!
I get all my relationship advice from biker gangs.
I sure wouldn’t want that woman for my only friend ! How can you even trust her anymore she, has no respect for your brother so I can’t see how you can even call her a friend . She sounds like a real user !
And remember, when you tell someone something that is going to make them really really angry at their spouse or partner, they may get angry enough to physically hurt or even kill that person. So if you’re going to tell, make sure you let your friend know so she can take precautions.
Give your friend 24 hours to come clean with your brother or else you are going to tell him. He needs to know that his best friend and girlfriend are both neither. You are probably going to lose your friend because of this, but that’s true whether you tell him or not. So you may as well at least be able to walk away knowing you were a good sister.
There’s a really good chance that the “girlfriend” is not the only woman the “best friend” is doing. Likewise, if she will cheat on one of them, she’ll cheat on both. And people like that tend to be less responsible with protecting themselves and their partners. You don’t want your brother stuck with an STD for life, or sweating out the next ten months, wondering if a baby is his or the other guy’s.
One way or the other this ends badly. The sooner it ends, the less time the situation has to worsen.
Then you and your brother should go to a good family therapist to try and find out why you have such lousy taste in friends/dates.
Odds are that he is going to find out sooner or later anyway and may find out that you knew about it. Will he be upset with you for not telling him when that time comes?
I say send a note, don’t sign it.
Excellent advice - I concur.
Because a relationship is not a low tire or an open fly. It could well be that he knows about being cheated upon but, because he would rather keep his current girlfriend than have fidelity, is pretending to the world his ignorance. Your telling him forces a confrontation he didn’t want to have.
It’s silly but it’s what happens when people in unconventional lifestyles are forced to look like they have everyone else’s. She could lose her relationship with her brother, her friend, and the one she’s cheating with, if s/he’s in their circle.
I’m not so sure.
Let’s assume he already knows and either likes it that way or at least tolerates it as better than the alternatives. OK, so OP corners brother in private and says “I hate to have to tell you, but your GF is cheating on you.”
So he responds with “Oh, I knew that. We both do; it’s an open relationship.” If he’s *really *open-minded he might add “Care to join us some time?”
Or he responds with “Yeah, I know. It’s a major bummer, but until I find somebody else, she’ll do.”
Or he responds with “Mind your own og-damned business! Do I look stupid!? Of course I know. We’re working on it and you need to butt out.” She says “Look bro, I was just trying to help. Have it your way. Sheesh.”
Damned if I see more than trivial downside to any of those options. And there’s lots of upside if the bro is actually an ordinary, decent, but clueless dude. Which is the 90% case.
For sure a smart person in the OP’s situation won’t spill the beans in a dramatic public confrontation that involves more people; that just invites drama for no benefit.
Exactly…
I still think a anonymous note is the way to go. But, you’d could never, ever tell.