I'm going to start with some questions.
1. Are you at odds with your sister or family for other reasons? Abuse ,narcissism , they enable others self destructive behavior?
2. Do you have a severe social phobia?
If the answer to question 1 is a yes. I get it, you should remove toxic people from your life and move on. I'd support not being involved with toxic, controlling abusing people, esp if they are family.
If question 1 is a no. Then you are otherwise a normal member of your family, and you should support family events, weddings, funerals, celebrations etc.
If the answer to 2. is yes or maybe, that is something you need to work on. The world is a social place.
My son had a severe social phobia as a young child, to the point he was mute around everyone but family.
We got him NON-MEDICATED therapy from a child physiologist. It included us modifying our parenting style to not reinforce or make his phobia worse. He was quite successful in dealing with this, to the point that as an adult he is quite comfortable with groups, and even enjoys public speaking.
If your entire reason for not wanting to do this is your social discomfort. I would be very clear about that, tell them, i am extremely uncomfortable in front of groups of people and its a phobia for me.
If they understand why, and they care about you, they hopefully will not pressure you.
But you need to do your part to get better. There is no reason to avoid any and all groups of people especially family, due to a phobia.
The phobia won't get better , in all reality it may get worse. Don't become a Agoraphobic recluse.
If its just a control thing for you, then you need to suck it up and support your sister on her big day, be glad she does not want you to be left out.
You need to make your decision and stand with it. Don't say "I don't want to" , then go get measured for a suit,( and be angry after getting fitted) that sends mixed signals. Stand on your feet firmly, be able to articulate why and be done. ( If its not family, you don't have to articulate reasons or anything).
If your are going to be a member of your family, support them, or give them an honest reason as mentioned above, so you don't just look like a jerk.
Either you are a member of your family, or you are not ( and maybe you are not for valid reasons).
Straddling a fence gets you a sore crotch. You have to pick a side.