We did this in December 2002 with grandparents - 2, parents - 4, aunt and uncles - 4, cousins - 2, brothers and sister - 3, and nephew - 1 (total 18 party members). We stayed at a grand villa and some studios at the boardwalk and it was very crazy but everyone must have had fun because when we go again next Christmas we will have 9 new party members!
Tips:
1. Plan some meals together - we did Whoop-dee-doo, Chef Mickey, Donald's Breakfastosaurus, Prime Time Cafe and Cape May Cafe. Next year we are returning to Prime Time and Chef Mickeys but we are adding the Luau, Crystal Palace and the Castle.
2. Make it clear that everyone is free to do their own thing. Go to enjoy spending time together but don't make it a requirement that you must do everything together. It all went much smoother this way.
3. We are going to try and manage a fireworks cruise this trip too. Ohhh - I can't wait!!
4. Take an hour out for a family portrait in the lobby of your hotel - you will find dozens of helpful photographers. We got a wonderful shot that I had blown up and given to everyone as a Christmas gift - it is a treasure having that and those memories, especially as a deeply beloved member of our family will no longer bounce up and down like a little girl to the entrance to the pirates of the caribbean any longer. It was worth every moment.
Have a wonderful time!