So this will probably be long too. It's a bad habit of mine. :(

I do believe her sister is also probably a bad person. But OP is not married to her sister and can easily not contact her further.

Please understand, the entire point of my suggestion is NOT for the husband to run off on his own and torment the sister in the name of his wife's honour.

It is to sit down with his wife and seriously talk about what it will take for her to feel she has had justice, or revenge, or whatever it is that she needs in order to make some progress in finding out exactly what it is she needs to feel closure here.

however if there is no solution she is willing to accept other than sabotage, then I honestly feel like, IF that situation arises, OP will have to pick a side, because idleness from an ally is tantamount to obstruction in the mind of the scorned and angry.

Like I said, if my SO delighted in using sympathy as a weapom, even to someone that wronged him, I would not want to be with him.

Well, I mean, ok. I'd still be fine dating someone who sticks up for themselves so long as I feel they are justified under my own moral code. I'd be more inclined to leave someone if they demonstrate to me that they're cowardly and unwilling to stick up for themselves.

And to be clear here I'm not talking about physical violence. Someone who says "yes" to everything asked of them because they're too afraid to be mean to people, or let people walk all over them because they're too shy to stand their ground, is a MAJOR turn off for me.

but hey, different strokes for different folks right?

Just like I wouldn't want to be with someone who was beat to shit and then turned around and calculatedly planned the perfect revenge where the other person is killed because holy shit that's a level of dysfunction that I don't want to associate with, even if they did deserve it.

Whoa now. I just want to be clear here that at several points throughout my comments in this thread here I have specifically said NOT to break the law. I have never suggested murdering someone. I really hope you didn't get that impression from my comments.

Up until this comment, you've talked about exacting revenge. It's much better advice to talk about it to vent and then not do it, so that's a good shift!

So here comes the long part....

First of all I agree. If you can de-escalate the situation without confrontation then all the better. I feel however that it's very unlikely that this is going to be one of those situations.

The problem is that there's too much happening that will taint the relationship if OP doesn't show solidarity.

I mean, if you're driving along the road and some ass cuts you off in traffic and flips you the finger out the window, you don't drop what you were doing to tail him waiting for an opportunity to ram his car into oncoming traffic.

You roll your eyes, realize you're a better person than him in every way that matters, and get on with your day. Five minutes later you've forgotten about it anyway. You make this choice because it's a random event from a total stranger who you'll likely never see again and even if you do, you probably won't even recognize each other.

Here though, the sister messed up the wedding with a suspiciously coincidental series of events that would lead me to believe it was 100% intentional.

There are no wedding photographs, no honeymoon photos, and every single memento that remains (luggage tags? unused plane tickets? receipts?) are all just reminders of how awful a day it was.

There's nothing good to salvage here, and the absolute best case scenario that OP can hope for is that she'll just pretend their wedding day never happened.

She has even suggested getting a divorce so that they can legitimately marry all over again, which would suggest to me that a simple renewing vows ceremony isn't good enough. She feels so strongly that she doesn't want the vows "renewed" in some hokey ceremony - she wants to start again and have a proper wedding day done properly and is willing to legally divorce in order to get that. And from that it goes without saying that I really doubt that she's willing to just forget the day never happened.

These are some seriously powerful emotions. And it puts OP into a situation where his marriage is potentially in jeopardy.

In the end, if nothing else works, he's going to have an ultimatum - side with his wife and cause chaos, or sit back and wait for this ticking bomb to explode. Once her sister's wedding photos go on facebook, with the cuddling and the smiling and the artistic couple-standing-over-puddle shots, and the whole family comments "You look great together! You look like such a happy couple!"

This will all rise to the surface. Resentment is very hard to clean away, and it won't e directed at the sister this time - it'll be directed at OP for refusing to help her stop this from happening.