I am reading this as that you react a little differently to social situations that other people. Do your family (and your sister) recognise this?
Weddings are quite intense for everyone in different ways. I think you should go, but maybe you should make a plan about how you can be there without it being distressing for you.
First, talk to your sister. Tell her that you are happy that she wants you there and that you will come but you need to make a plan so everyone has a stress free happy day.
Your sister will be busy being the bride on the day. Can any of your family who will be there on the day be your back up? The people who you can stick with, or who can come rescue you if you look like you are stuck making small talk. Figure out your top concerns and then if you have someone there who can help, make a plan with them about what you'll do.
Having a job to do can be helpful as it keeps you busy and means you don't have to do too much socialising. Usher? Be responsible for gift table? Decor?
It sounds ghastly but perhaps have one or two 'small talk' things to say to people. A good tip is to ask a non important question about the person and let them do the talking. www.google.co.uk/amp/s/www.forbes.com/sites/christinapark/2015/03/30/an-introverts-guide-to-small-talk-eight-painless-tips/amp/ It's ok to plan in advance. I am an extrovert and find small talk easy, but I do notice I have my tactics. I observe things about people. also if you are making small talk at an event like a wedding, it's ok to say 'it's been nice to talk to you/meet you/catch up, I'm going to find relative/buy a drink/ take some air' and that doesn't mean the person is boring. If it happens to you it doesn't mean you're weird.
Also it's ok to be a bit odd and immature! Some people get a bit thrown by it but many people will be able to make some small talk. I agree stick with the ceremony and wedding breakfast. After that it will all be a bit merry and you can wish your sister full congratulations before slipping off.
I think you'll be able to make it work for you, it will just take some planning. Don't not go, I think that would be a mistake.