After experiencing a family gathering with 14 of my in-laws a few years ago the best advice I can offer is don’t, not unless you are all on the same page so to speak, let me explain.
One of my brothers-in-law is a Disney fanatic like I am. We attempted to plan a trip that fit everyone’s needs and we failed miserably. Everyone in our group had different requirements as to lodging, dining etc. We spent weeks in an endless string of e-mails, and phone calls to set things up. We are all in about the same financial bracket so we tried to convince everyone to stay at the Wilderness Lodge but mother-in-law and sister-in-law insisted on a moderate( Port Orleans), another brother-in-law wanted a value (Pop Century) and the 3rd bother-in-law wanted the Dolphin.
It was the same with the dinning and with what parks to go to everyday, etc. In the end my family stayed at the Wilderness Lodge as did Disney fanatic brother-in-law and his family. We spent most of our time together and had a great time. We only got together with the rest of family twice for park touring, a day at the Magic Kingdom and a day at DHS.
We only did two dinners together also, one at Chef Mickey’s and one at Whispering Canyon.
That night at Whispering Canyon my mother-in-law, sister-in-law, and bothers-in-law all stated how beautiful the Wilderness Lodge was and how they wished they stayed there. Disney fanatic bother-in-law just looked at me did the face palm and shook his head as if to say “yup this is my family.”
I hope your grand gathering works out better than that!