I was supposed to be a bridesmaid at my sister's wedding at the end of July, but my EDD is August 7th.
I flat out told her over Skype that I wouldn't be able to make it to the wedding. She was extremely disappointed for about 10 seconds until I told her the reason why.
Now, her husband-to-be is planning the telecommute for us guests who won't be able to make it...they are setting up a laptop with Google place or Skype on it for us out-of-towners to attend. My sister is thrilled and totally understanding.
Just let her know, and maybe the excitement of impending auntdom will outweigh any disappointment.