I was v young. 5 of us shared a student house, 2 supposedly committed couples & a single bloke. I'd just got engaged to my DP. Out of the blue my DP and the OW went off together, leaving the 3 of us utterly devastated. The single bloke was timid and idolised my charismatic, hugely entitled DP. He was almost as broken up as we abandoned lovers were.
Massive heartbreak for me and the OM, and a total shock. The first time I saw them together at the library I fainted down the steps like a Victorian miss. It was a huge deal for me.
My bastard DP left all his stuff behind, asking me to pack it up and send to their new flat just a few streets away.
So: I ripped a handful of pages out of the back of all his books, poured bleach on his dark clothes, black dye on his light ones and wrote "Fuck off" on all his vinyl albums with a pair of scissors (God he loved those albums!). Then I put the albums back in their sleeves, dried the clothes, packed it all up neatly and got the whole lot delivered. Slow release revenge.
During the rest of the year I had lots of comforting sex with the 2 remaining blokes (2 nights a week each kinda thing). It was lovely, and healing for all of us.
And when I discovered that my hair-trigger ex had run off with a woman who required a solid hour of oral sex before she'd permit penetration... Well, it makes me smile even now... She eventually joined a lesbian commune.
But, OP, I can't see any way you can ruin this wedding for your sister without being almost certain to hurt non-combatants, especially her poor innocent daughter. I'd leave it. Be the big, kind, grown up one. And fantasise. 