Your problem isn't your sister's wedding, it's your fear of coping with crowds.
If you don't want to be crippled by anxiety for the rest of your life, I suggest you seek therapeutic counseling immediately, and work on your problems.
As far as attending your sister's wedding, of course you are expected to attend. Your family would think you're crazy if you didn't. If you're not crazy, you should be able to cope long enough to share in your sister's important live-changing event.
I like AlohaHana1's idea of taking something along to calm you down. Bring a small, fiddly toy, a shiny object, some knitting or crocheting, paper clips to fasten and unfasten, your phone and some discrete earbuds to listen to music or a book, or whatever will occupy the edges of your mind so that you can watch the event without cringing fear.
But see a good therapist. There's no reason you should go through life crippled when help is available.