This. We don't know - she may be in a really tricky spot and could be hating the fact that she has to send the message. Is she usually a troublemaker? If so then yeah ok I would get upset but if she is normally a pleasant person then it doesn't sound like she is doing it to be horrible or disrespectful.
While I don't agree with what she has asked, weddings can be hard enough as it is when you through two families together, perhaps she is trying to avert what she knows could lead to trouble. I would go and be as discreet as I could and know that you have done everything you can to keep the relationship intact. It's only one day after all. If she is just being an insensitive troublemaker then this will become evident in the near future anyway.

