This can be so dramafull (yes, I made that word up). We asked everyone closest to us and they said they would be there. Then we booked...then they backed out!! Then we planned without them, and now they're coming!! Granted we're still 6monthes out.
I gave them a deadline that I had to have reservation #'s by and if I don't have them, they're not coming.
The way that disney has it set up, they won't really be able to just show up with out you telling your planner, so I wouldn't worry too much about it. But, remember that your having your wedding for you and DF not your family, no matter how close or not close they are.
Good Luck!