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Well of course he would.  No one likes it when everyone knows what a bad boy they've been.  That's why grounding was always SO much worse to me than a spanking.  Spanking is done and over with, no one the wiser.  But with grounding you have to tell your friends WHY you can't hang out... then they can all think you're a moron by speculating on what you did... or didn't... chores, grades, etc.  Me, I'd tell the truth.  If he was really all that concerned about being humiliated he wouldn't have opened the box... especially knowing that it could lead to divorce... I mean REALLY what did you think would happen then?  Yeah, I imagine he's quite pleased that you just kept your trap shut and let him get by with it.  He got to play around, YOU got humiliated, and no shame for him.  Pretty obvious who got the short end of the stick on that one.  You announce what happened and not only is everyone going to know what a punk he was THEN, but also they'll be wondering how much a punk he is NOW for making you attend.  Also will wonder how stupid his sister is for keeping that tramp right next to her groom.
Yeah, not hard at all to see why he and his whole clan would prefer you to just shut up and take it.  And like SO many, if you don't just shut up and take it then they'll use guilt and make it Your Fault... not only his cheating, not only her predation, but also for ruining the whole wedding.  All your fault.  But, same note.  If that day, before, or after... EVER R makes a move on the groom (or any other married man you know) then it will also be your fault because you didn't warn everyone about what that snot was up to.  
Some people are users, abusers... some keep their heads in the sand... some enjoy scapegoats... when you've identified one it's best for your own mental, physical and marital well being to steer clear.
Of course, I loathe confrontations so that's part of it.  But also, I'm VERY blunt.  If I was in your shoes and DID attend there is no way I could hide my feelings which would just miff SIL and Co... and I'd be hard pressed not to walk up and call that tramp like I see her in front of everyone and be done with it... once the cat's out of the bag then that part would be over.  Everyone could deal.  Get over it, OR prepare for attack from the wench, and they'd know precisely why you didn't want to be there... now some would be hoity about it, but I bet you a number of the married women would be behind you, and wouldn't let their hubbies out of sight either.  
You need to remember that YOU are NOT the bad guy here.  YOU didn't do anything wrong.  The rat fink and the tramp are the ones who purposely caused you pain, and in tramp's case not only you but another marriage too... and that's just the ones you know of.  There's really no telling how many wives are in the same spot at you... keeping their trap shut... hurting but dealing... you might not be only the second one... talking about it might get them talking too... might help them heal from the wound... AND it might just get this tramp out of your group for good.  For those two reasons alone I'd have a hard time keeping mum... and thus for peace's sake it'd be better if I wasn't present.
Of course... if I was really peeved I might just act all nice and then make a toast at the wedding stating all that out of the blue...  **part of sentence removed. Please remember to use language suitable for all ages**.. but really would depend on how mad I was, and how much I really cared about the other wives... have to think about whether it's worth it or not.