I would not sign on to book anything, collect money, or do anything else that I could get burned with. For a group this size, it's really not practical to do tons of ADRs (imagine always having to wait for Uncle Louie's family!).
What I MIGHT consider doing is sending out a newsletter to the group, with information like this:
(1) potential dates for a trip
(2) rough idea of hotel/DVC costs, with different price points (1 value, 1 moderate, etc.)
(3) a couple suggestions for 1 or 2 group ADRs--Chef Mickey's? Hoop-Dee-Do? Luau?
(4) suggestions for group activities, such as water mice or mini-golf
(5) a list of popular websites for more specific information
Once some of the big issues are settled--who is actually going, what dates, etc., you can then mention your itinerary, with some ideas of your favorite rides and attractions.
In general, though, I would be very wary of taking over and doing all the planning. I especially wouldn't fall for the whole, "plan it all for us!"--that's just lazy on their part, and it sets you up to get blamed when they, say, don't like to make rope drop or are the one person on the planet who hates Soarin' or whatever. Use phrases like, "what works for me is...", rather than saying, "OMG, you just HAVE to ride Small World!"
All that said, I think this could be fun, but only if the whole gang has a sense of humor, sense of adventure, and are the type who roll with the punches. Getting a group that big to agree on something is going to be tricky. If you're an early bird and your brother is a night owl, work with it--maybe meet him for lunch in Epcot one day, rather than trying to get him to rope drop (and then getting frustrated when he doesn't show, and you're stuck at the turnstiles waiting for him).