I usually suck it up but i do think it's different being immediate family rather than randomly in the background.
Correct. When you're immediate family, it's far more important.
If you don't go, it will be seen as a snub and a rejection. It will hurt people's feelings, hijack your sister's happy day and make it all about you. This may not be your intention, but it's what will happen and how it will be seen.
People will not think how introverted and solitary you are. They will not think how deep you are. They will not wonder how difficult it is for you and how complex and rare a creature you must be. I don't like the fact that posters are calling you a brat and a selfish attention seeker, but frankly it is how you will appear and it's what people will think. And your sister - who sounds lovely, by the way - will be very very hurt and it will cast a shadow over what should be the happiest day of her life.
I know some women sincerely don't care about fashion and style and femininity, but in my experience, the ones who make a big song and dance about how much they hate dresses and hairstyling are usually suffering from cripplingly low self esteem, and they are rejecting the concept because they don't think they could get away with participating in it. If that's true of you, and I suspect it is, your sister may just be trying to encourage you gently to try the things out in an attempt to help you. She might be misguided in that, but it sounds as if she is well intentioned. She's also tried to get you to go on a city break and go for run - it sounds as though she's willing to try lots of different things to bond with you.
Stop making it about you, stop creating unnecessary drama, and just put on a smart suit, go, smile and be bored for a few hours and then you're done. Is this the worst thing you've ever had to put up with? One evening of your life for someone who clearly loves you?