Quandrea has great advice. There are 23 of us in our 'immediate' family now and we all go to WDW at least once a year. We stay at the same hotel because we like to visit each other and being close lets us do that.
On arrival night, we eat dinner together at O'Hana and on the first day we make a real effort to ride the boat to MK as a group. After that, we meet up in smaller groups and don't try to be joined at the hip. We usually schedule some special meals together (Via Napoli is great for a big family meal) and usually most of us go to a character breakfast, either at Cinderella's castle or in Norway.
Other than that, we go our separate ways. Sometimes we exchange kids and someone takes the little kids and someone takes the big kids and the adults take turns babysitting. It's nice with a flexible, big group. My grandson loves AK but that park's not a favorite of the others, so sometimes he and I go there twice.
I love to take my Daughters-in-law to a fancy dinner by themselves while the Dads and Grandpa take the kids to do something and the guys like to hang out together, too. The key is to be flexible and not feel you have to be together every minute as a group. It just doesn't work.
Enjoy your trip, TravelMommy.