If OP doesn't, she will..

man that'd kill me not being able to get my best friend out of stuff like that but I'll admit I wouldn't want to ever believe that my other is like that -_- hope less romantic

Not gay in a three-way!

True, and then he'll forgive OP for trying to help him. Just give it time OP.

If I was OP, I'd tell his friend to take his forgiveness and shove it where the sun don't shine. Serves him right for being an asshat.

Why should the friend forgive OP? All he did was try to help. The friend is the one who needs to apologize.

I have a feeling we will have an FML that reads, "Today, my best friend told me that my girlfriend was cheating on me with my brother. I didn't believe him and even stood by her until I walked in on her and my brother." FML

There are some things you just don't do. You don't share a toothbrush, you don't use a stranger's deodorant, you don't not flush the toilet at someone else's house, and you certainly don't heartlessly cheat on someone with their sibling and then smile at the person who tries to reveal the affair. That is one nasty snake of a woman OP's buddy has o.o

I guess it must have been hard from OP's friend's perspective because he has just been told at least one of the three people who are meant to be close and care for him, is doing something awful. I imagine it is easier to blame one person than your girlfriend and your brother.

@20 To be fair, and I'm not saying what the friend did was right, but who would really want to believe that their own sibling would be shitty enough to fool around with your significant other? Without proof most people wouldn't believe it. But the friend is still an asshole for treating the OP the way he did.

Talk about doing the dilemma. movie joke

Now he'll find out months from now and feel like even more of an idiot for not believing the friend that tried to warn him months ago.

#8, that has got to be one of the dumbest things I've ever read on this site. And that's saying something!

If he doesn't believe you now, doing that will definitely get him to believe you. He may hate you for it, but a cheating cunt is still a cheating cunt.