Hi Diane, so sorry you are SO not looking forward to this wedding.
imho I would suggest you let your sister 'run the show - be the boss - organize her heart out'. You just sit back and ENJOY doing nothing that you don't want to. Enjoy those grandkids, your son and d-in-law. Be lazy, don't (try not to anyway) agonize over it all. You will only make yourself, spasms,etc. worse. I can understand it must hurt to hear your parents say "she lives here you don't". But they must be used to her organizing ways, may even depend on her for that.
I know it's easy to say but harder to do. So lots of deep breaths, count your blessings and be determined to just R E L A X.
June in Toronto