Oooh snap.... Well its not like you knew.... But he's a pig. At least you can now warn your sister before she marries a prick! = Its not your fault. <3

that isn't **** your life, thats **** your sister's life. I hope you showed her his phone number the moment you saw him so there is no doubt this is the story and its not some made for revenge ploy

I agree with #1 100% I hope you told her what a prick her fiancé is...

This is not an FML. This is a very good thing. You now can save your sister years (or months) of pain.

You know, this sounds strangely similar to a situation in Dan's Real Life. I'm glad you found out what a creep he is, for both your and your sisters sake!

What a dirty cocksucking bastard. You should slap yourself on the back and say congradulations because you just saved your sister's damn life. Her would-have-been unpleasant destiny anyways.

Now hold on... True, it is a bit shady, but it's not like he already cheated on the sister. Marriage is a big beginning, but it's also the end of something, his life as a free man ;-) Maybe the flirt was just that, a flirt. The exchanging of phonenumbers is taking it a bit far, but how many men never follow up on that? 80%? ;-) Well, even if just 1%, he could have been that one. I know my boyfriend sometimes flirts with other women, I do too (with men then) when I'm out with the girls, it's exciting/fun and tells you you're still desirable, but it doesn't compare to what I have with my boyfriend. I have never cheated on my boyfriend and I'm convinced that he hasn't and never will either. The fact that he accidentally flirted with his fiancees sister may only show he has a specific type ;-) (as sisters are often alike) and does not necessarily mean he'll be a cheater. So FYL for flirting with a nice guy, and finding out he's not gonna call you in a rather weird way (cuz he's taken). At least you'll know why he didn't call ;-)

That would have made for an akward situation at dinner, but its your sister that has the real problem here, I hope you talk to her about her man ASAP, or she's going to have her heart broken.

I just physically dont find it possible to flirt with girls that arent my girlfriend, it just doesnt do anything for me but make me feel bad you know? I love my girlfriend, and I dont have the predominantly girl's need to feel desirable. So much of female desirability has to do with flirting and recieving attention. Guys just dont operate like that. Outside of performers, guys dont really crave attention all the time to reaffirm ourselves like girls do. Its different with guys...when we flirt, its not cause we're just being "nice", and reaffirming ourselves. Thats not even a door Im willing to knock on, open, walk into, get on the bed...etc. Its too easy to get caught up like that. I only bark up one tree and thats my girlfriend. Your talking about a quick road to distrust, distraction from the relationship, and heartbreak. Not a good investment of time and energy.

Let slip to your sister that you saw him talking to a woman there and give him his number. Let her do the rest. It will be more impact if he tells her that he was hitting in her sister. If you tell her then she may end up shooting the messanger.