Not to be dismissive, but welcome to the wonderful world of planning a destination wedding!
This has happend to a majority of us that have already gotten married. If you just browse the "just venting" sub forum, you'll probably find 2985735 similar posts!
The bottom line is this, you have done the responsible thing in being up front with your guests regarding your plans and the costs. You have also given them ample time to plan. As hard as it is, you need to move on.
Those who want to attend your wedding will find a way to do so. Some people just like to complain. If it's not the cost, it's the resort/country, the dates, the whatever.
My DH's brother kind of whined about the cost, but in the end he was there because it was the right thing to do. Only DH's immediate family came and lots of mine. All those who did attend, kept talking about how wonderful it was.
The most important thing to do is paln what you and your FI want. This is a special time for the two of you, not anyone else.
Good luck!