It sounds like they're going far out of their way in order to be shit to you and make themselves feel like they're better somehow, so it's reasonable to be pissed.

Hopefully you'll get the last laugh when they have to pay for two weddings just to be petty.

ed: I'm not actually sure what you could do, sorry. Try to focus on your own wedding, and planning your own future, is all I can come up with. Weddings are huge elaborate messes with lots of familial drama, so if anything this might at least be the last time you have to deal with their fuckery on this kind of scale.