I'm very much the scapegoat iny family and my dsis is the golden child so I absolutely understand the dynamic. However honestly you sound like me 15 years ago before I grew up. I don't say that to be mean but I do say that to highlight this has nothing to do with a wedding or a holiday and everything to do with railing against the dynamic which trust me is totally and utterly pointless.
Regularly I would make things a massive issue in my head because it boiled down to another thing that showed how unimportant I was. It came to a point where you realise that it's making you the unreasonable one. The childish one.
To a certain extent its justified. Its crap being that person and being highlighted in a 100 different tiny and big ways that you are considered less important than the other.
However it made me a person I didn't want to be. I tantrummed about it all. I insisted on pointing out every petty thing my dsis did. Until I realised I simply didn't want to be that person.
I made a clear decision. I would do what was actually right with me and mine and everyone else could do their thing. I ignored or accepted the winging when my choices didn't match up with their expectations of me. I simply separated. Dsis can do what she chooses and I either choose to engage if it suits me or I don't.
It honestly is as simple as that. You will never win this. They will never turn around and say "Yes you were right we have been unfair" . The only thing that happens is you get into a tizz at the unfairness of it. You get stressed. Often you end up behaving as a points scorer out of frustration and it simply ends up with you being the actually unreasonable one railing at other people choices. Of course it is actually a reaction to years of behaviour but it doesn't matter in the moment.
Seriously , stop caring , stop wanting their validation, you won't get it. The only thing you can do is make your own choices and stand by them. What they all choose to do or say is none of your business..
I promise from experience this is a battle that will make you someone petty and nasty and you don't want to be that. I get it, I truly do and I have huge sympathy for the frustration and anger but it really is only hurting you. They don't care , not really. The only person it affects is you. You can absolutely choose to simply dp what works for you. That's it.
If you don't want to go. Simply don't go. They will rail and winge that you are selfish you can simply choose to know that it was the right choice for you and that's it. The unreasonable bit is claiming she shouldn't have booked it. That's not your remit. Choose to go or not go and stand by it.