Good tips. We had a very Magical extended family stay for my DS's 10th birthday in 1998. There were 16 family members and 5 friends.
Ahead of time, we asked who wanted to go, with plenty of time for everyone to save up money - I think it was about 1.5 years ahead of time.
Then we let people know where we were staying & the different options & price ranges available. We settled on Fort Wilderness Cabins, with one family in a their own camper & one couple at POR (then DL) Being in the same resort was easiest for popping back & forth between groups. Altho we did ride bikes over to DL to visit there also.
We told people they should treat this like their own vacation & do what they wanted to. We scheduled a couple group events on DS's B-Day. Other than that, we seperated into either family groups or small groups. For instance, one day, I took my youngest nephew & my parents took my 2 older nephews so my brother & his new wife could go for a romantic horseback ride. One night, we had dinner with my step-son & his wife.
The only time we did any touring as a large group was right after the birthday breakfast, we did Buzz (just opened that day) and off to Splash for Family Ride Photos. Other than that, it is impossible to keep a large group together & it should not be expected. As someone said above - no expectations & go with the flow are key attitudes.
We have traveled with my Parents 3 times so far. The first 2 times they shared a FW cabin with us & another time they stayed with DS & I at the BC until DH arrived. We stagger the trip so that part of it is core family time & part of it is including the parents. We always offer my parents alone time, which they rarely take, but do occaisionally. For the most part, everyone relies on me to do the planning & they all seem happy to go along. I find out ahead of time if there are any must do's for them & I try to plan things that I know are special to each person, like a favorite restaurant for DH, a relaxing stroll in WS for Mom, a boat ride for Dad....
Again, the key is that we are all entitled to break off at any time. We never all have to be together every moment. We have found that essential to our core family trips as well. DH likes to have a more relaxing WDW experience, while DS & I usually want to go, go, go every second. So, sometimes, DH will go to the resort & relax & meet up with us when he is ready. Sometimes DS (16) wants to swim, while DH & I go wander around WS & we meet up later.
Unless you can't stand to be around your family in real life, you can have a marvelous extended family trip to WDW if you go with no expectations, relax & allow folks their own space. I look forward to doing more extended family vacations in the future.