Ouch, nasty is all I can say...
Your FSIL is probably thinking that the Mayan wedding is going to be beautiful, and there's no way she can compete with just a local wedding, so when she found out that you would be doing it, she all of a sudden thought "Why can't I expect my friends to pay to go?? She is!"
I know if I was in the same situation as your FSIL - I'd be very disapointed that someone else was going to go with the wedding that I wanted, but don't worry, I'm on your side, she had the chance to go with it, and decided against it...
Hopefully she'll either A) come to her senses or the whole family will rally behind you, and tell her that they wouldn't be able to go to her wedding, because they'll be at yours.
Just make sure to keep us updated as to what happens!