this is something only a genuinely bad person would do.

A genuinely bad person bullies people around them because they can. A genuinely bad person plans, and executes the ruining of their sister's wedding day.

If you think that retaliation against an unprovoked attack makes someone a bad person then I honestly don't know how to relate to your world view in any capacity.

Also with regards to living with other people, you're missing the point. Nobody else will know. That's how it works.

How do you envision this happening in my head?

You hide a bucket of red paint in the rafters, signalling your ninja henchman accomplice at the perfect moment during the first dance to drop the bucket on her head, drenching her and ruining the dress.

As she stands there screaming you leap out from the shadowy corner, "ah-ha!" you exclaim excitedly, "It was I who enacted this perfect revenge plan!"

The family and friends all laugh, pointing at her and reveling in your masterful execution as you strap on your jetpack and blast off into the horizon as Beethoven's 5th plays over the sound system.

Is this how you see it happening?

I'm not going to give examples here because frankly, I'm already tip-toeing on the line by even suggesting something other than the mundane "marriage councilling and/or divorce" that shows up any time there's a relationship based conflict in this sub.

But let me tell you exactly how I'd deal with this situation, and how it's playing out in my head.

This poor woman had her wedding ruined and that isn't going to be fixed by sitting down with an overpaid college drop out asking her "tell me how that makes you feel?"

Expecting her to play along with that is an insult to her intelligence, and brushing off her desire for revenge invalidates how she feels.

Partners should be there for each other. And that means being supportive. You want revenge? Fine, let's sit and think about how it'll pan out.

First of all, that is therapy in and of itself. She gets to feel connected to her husband who's helping her scheme. She gets to enjoy imagining the intoxicating fantasy scenarios that the two of you create together.

She doesn't feel isolated and alone any more, she has someone who understands and wants to help her. Someone to share her outrage with and that is often what people really need to help process a traumatic event.

They just need a cheerleader to stand beside them and say "Yes you're right! Your sister absolutely DOES deserve to be set on fire and kicked to death! but as much as I agree with you, I don't want you to go to prison so let's see if we can find something else that we could get away with!"

And from that moment, you can begin to de-escalate the situation.

Second of all, it gives her a chance to seriously talk through her feelings and in doing so, might come up with a more realistic solution to the problem.

It helps get to the real root of the problem. Once she feels like OP is on her side, and is more willing to open up with how she feels it gives him an opportunity to try and identify exactly what hurts her.

Is it the finance? Then let's see if we can figure out how to get the money back. The catering gave us food poisoning so we should demand a refund and sue if they refuse. The photographer didn't actually deliver any photographs so let's look at the contract and see if we can get compensation or a refund.

Oh but that's morally wrong since his camera was stolen? Then let's call the police and report it, and let them investigate. I'm sure the sister, I mean unknown thief, will get caught if we cooperate fully.

Oh and the flights, let's see what we can do there to get a discount or refund.

Is it the slight against her ego? Then let's get some validation by speaking privately to family members to see how they feel. We can use their responses to pick and choose who we keep in our lives from this point on. Is gran-gran's complete unawareness of what's going on around her sufficiently damning to go no contact? What about the loud uncle that plays favourites?

Perhaps it's the humiliation from her sister? Then listen and realize that this has (maybe) been going on for a long time and it's time to make the sister stop hurting her. So let's look at what avenues we can take to get the sister to stay away from her. Talk to the family, see who noticed what happened, then go public once you drum up enough support.

Planning revenge with his wife isn't about figuring out how to get away with assassination and where to hide the body. It's about showing her that yes, her feelings matter and identifying a bullet-point list of issues that she feels need to be addressed for her to be happy again.