Consider how she must feel having to even ask this of her future SIL? She must have some ultra conservative people who she knows will get highly offended and upset if she has decided that this is the avenue she has to resort to. Imagine how she would feel on her wedding day when she is trying to relax and be happy if instead she had to be on edge and nervous that people were getting upset or even possibly angry about something. I know as a bride I wouldn't want that and I would want to minimise the possibility of this happening. She has done that in a discreet and reasonably polite manner. She hasn't shouted it from the rooftops, she hasn't gotten angry or made a scene, she has simple made a request and outlined her reasons behind it.
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.