Wedding

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Seconded

I think issue here really is your willingness to connect with your sister prior to the event. Bring really open and saying 'im struggling do much with the idea of being at the event that it's making me ill'.

She may perhaps then be open to discussing alternatives. Maybe being there for part of the ceremony and then again for the toast and speeches? Both situations would require less socialising, at a guess.

Open the dialogue on it early though, very important

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Just wanted to add (in case it may help in some way), I attended a cousin's wedding a few months ago that I had been having second thoughts about attending because I knew I wasn't in a good place mentally and emotionally. This was something I had made my cousin aware of prior to his wedding.

Well, I attended the wedding and am glad I did. My cousin would not have invited me if he and his now-wife hadn't wanted me there, and I know I would have found it difficult to forgive myself if I had not been there. Whilst the prospect of large social gatherings tends to fill me with dread, it wasn't as bad as I had feared it might be. The whole event was quite relaxed anyway, with plenty of opportunities for me to escape for moments of quiet time away from the other guests. 

As others have said, have a conversation with your sister and see if a compromise can be reached. 

The imagination is worse than what is real.  You’ll be fine.  Use your imagination of the future as follows: Imagine the whole community here and any ideal persons you would like to going ahead of you into the event.

That's great to read. Well done to your sister, and you. 

I agree with other commenters about talking to her and explaining what you find hard. At my sister's wedding, she made sure there was a quiet room I could go off too when it got too much for me.

Very solid advice.  I am in support of this.

But I understand all too well why. When I said ‘all the reasons’, that excludes the dress thing! 

I’ve been stepping around this thread for two days as even the word wedding makes me sick for all the reasons you mention. Sorry it’s stressing you out. 

Great advice as always. 

Hi, is your sister understanding of you being autistic? Have you told your sister how stressful going to the wedding would be? 

Hopefully she will understand if you feel unable to go or if you think going will cause too much stress. 

Or if you ‘wanted’ to try going you could talk to her about some of the things you are worried about (dress code, too many people  etc.) and come up with a compromise like other people suggested

If you don’t go, you could even suggest to your sister that you do something at a later date together to ‘celebrate’ the wedding- like a nice meal together just with your sister and her husband or something even more low key. If that is something you felt up to and wanted to do. Maybe that could be a good compromise because you would still celebrate her wedding but in a more relaxed and enjoyable way for everyone. 

I hope you find a solution!

This is all new, so you need time adjust to what has been asked, so give yourself time to process it. And when you have, consider the options, and if you have them, discuss them with your sister. Of course she wants you there, but she needs to work with you. 
You could dress smart casual, a top and trousers perhaps, which is a small compromise. You could attend just for the ceremony and then leave. There’s much less chatter and social interaction at that part. The meal and after party will be the ‘noisy people bit’. 
As for travel, you could take a travel sickness medication, or go by bus or coach.

Just think about what you can and can’t manage. 

I would start by stating your problems openly to your sister, then aim at a compromise. I'm sure that if you asked to not be involved for longer than you can stand, this could be accommodated. If the only way you could be present, is to slip in at the back, after the bride has entered the room or church, and then slip out before everyone else leaves, this would be preferable to your sister than your complete non-attendance. 

I can understand why your sister would like you to be at her wedding, but I can also understand your reasons for not wanting to attend.

HP35's suggestion of a polite reply declining the invitation seems a good one.

Just to add... Unless one is asked to be a bridesmaid, or there's a strict dress code/cultural/religious reasons, there is no rule that states female guests must wear dresses at weddings.