This totally sucks & I'm sorry that you're going thru this. Maybe this was her last ditch effort to try to get you to change your mind & have your DW in Jamaica. She probably did it thinking that she'll make you feel guilty & you guys will fold.
Since you've been together so long & you've always gotten along, I can't imagine why she'd have a problem with you now. You said maybe its an island thing, but maybe its just a "mother" thing.
For instance once we got married I noticed that my MIL sometimes try's to put DH in awkward situations & trys to see whose side he'll take when her & I have differing opinions. Luckily DH does not play into that. I'm his wife & it is what it is. We make decisions together & don't let her come in between us. It's like she tries to make him not forget that she is his mother & everything should go her way despite the fact that he now has a wife. Maybe this is what your FMIL is doing as well.
Yes, your FI may be hurt but if he's willing to go on without her being there & he's happy with what you chose, then try to stay strong & continue to stay with the location that you chose and do not let her steal your joy. This is your day, not hers. You will have wonderful memories of your day & in the end the fact that she wasn't there to share your joy won't even matter. She'll be the one who regrets it & ultimately it will be her loss.