I planned a trip for mere nine people, but three family units, mine, my sister's and our mother (She financed it also, like your case).
First thing I would say is that you will have a great time. Disney Cruises do have something for everyone!
Second thing I would say is try to set a few ground rules: (Maybe with your Grandmother's input):
- What times do you want all the family members together? Dinner, probably. Other times? Shows? Breakfast? For the 9 of us, we all ate dinner together, but the rest of the day it was mix and match. Different people did different things in different combinations. That way, you saw everyone, but you also got to do the things that mattered to you.
- Excursions. These are a MAJOR additional cost if you choose to do them. We chose to have each family unit pay for their own excursions and do what they preferred for them. Sometimes that meant 9 of use together, sometimes it meant two or three groups.
- Specialty restaurants. They are great, but they do cost extra, and they are adults only. Decide ahead if all of the adults will do these or only some of you. We chose to skip the specialty restaurants so as not to miss any of the Main Dining Rooms.
- Kids clubs. To me, this was a great chance for the kids to hang out with their cousins and for the adults to be together (or alone). But not all kids are into them, and some parents don't want to "miss out" on time with their kids, so figure out what the interest level of your group is for the kids clubs.
Next thing I would suggest is to find one designated contact person per family. If you keep trying to talk to 22 other people about every detail, you will go crazy. Have each family pick one person and then talk to that person about what you want to do.
Other notes: We enjoyed having connecting rooms, so that, when we wanted to we could go into each others staterooms without going into the hallway. We had verandah cabins, so we could connect that way too. It was great fun.
Also, for such a large group, the Navigator App is great. You can see what activities are coming up on which days, you can look at the menus for the coming days, and most importantly, you can text each other (for free) to help you meet up with various parts of your group throughout the day. Remember, you don't have to have a first rate smart phone to use the app, even an old smart phone with no cellular plan that can access wi-fi will work. For that matter, my niece used a old iPod touch that had wi-fi access. You can set separate "threads" so you can talk to as many or as few people in your group at a time as you want. I had one "thread" for just my wife and kids, other for all of us, and a few others for like just the adults or just my mother or sister or wife.
Planning this is a big exercise, but it can be a lot of fun too. I felt like my vacation went on for a whole year during the planning, and I talked with my sister more during that time than for a long time before or since, so that was a nice bonus.
