Thank you so much for all the replies!
It's great to hear others' experiences with a large family group.
We have definitely decided against renting a house offsite (which is good because DH & I didn't want to do that anyway). I do think it would be fun, but I think we will do it somewhere else for another trip (maybe going to the beach or something where we would really be "hanging out" instead of trying to do Disney).
I spent 7 hours in a car with my parents and one sister yesterday going to a funeral, so we talked a bit. My parents have been on 3 Disney trips, so they also have a good grasp on what would work best for everyone.
My parents would also prefer to stay at a moderate and they really like the dining plan (they have had it every trip). I will just have to talk to my other two siblings and see what they feel comfortable spending-- if a mod would work or if value is the extent of their budgets.
As for "togetherness"-- I think less in the parks would be best. I love my family, but DH & I like to do things alone with our kids and I just think it I would be too stressed and anxious trying to keep track of everyone and dealing with the wants/needs of that many people. I think it would be a constant stream of who needs to eat/drink/potty/take a picture/look at something in a shop/etc. There are a few family members who are way more high-strung and a few children who have absolutely no rules/discipline, so I just don't think trying to stay together would be a good idea for anyone.
We will likely wind up doing a few things in smaller groups, maybe just my brother's family and mine going to AK for the morning, my parents doing a character meal with the two families with pre-schoolers, etc.
I would like to spend more of our "all together" times at the resort-- let the kids swim/play, watch an outdoor movie, get pizza, have a few drinks, etc. And I think it would be nice to meet up for a few meals (which is why the DP would be good).
I have never done photopass, but I think it would be great for this trip. It would be really nice to see pictures of character meets and other times that we may have split up and not seen in person.