I was the "planner" last year for our group of 10, since I was the one that travelled more frequently to the parks and knew more about it. I only had 5 months to get it all working, so in that aspect you're more ahead than me.
Since you have enough time, I would write down all of the things you think you can show your family (hotel accommodations, tickets, food, prices, etc.) Get them all together at least once and let them know you want to get their opinion and get things going. That's what I did, we knew we were staying onsite, but I presented all of the possibilities, prices, number of days and we would discuss what would be better for all of us. After we all decided on the hotel and number of days, they sort of let me plan the rest at my discretion, but I would do the same thing as the start for fastpasses and restaurants you're planning on going, or just give them a list for them to give you their input. If you have the time, I would present them all the possibilities (even dessert parties and such, some of our family didn't do it, but we planned accordingly for them too) and just at least a few times get them all together to see what they think, if some of you want something and the others don't, plan what you can so that everyone is happy but taking in consideration that each person/family paid for their part too, so that you're all satisfied.
My biggest tip would also be plan on getting park hoppers. With being a few of you, sometimes at the end of the "park day" someone might want to catch a few hours in another park or even dine in a different restaurant once or twice, but still do the rest together. Also, take a lot of breaths and remember to enjoy it all! My biggest regret with my trip was that since everything was "on me" I stressed a lot about making everyone's trip perfect and forgot to take a breath and enjoy it myself!