What @broadwaybaby123 said. Also, if your group is that large, I suggest you work with a travel advisor who does cruise groups. They can set up a special webpage for family members to get all the info, provide information on their needs, and facilitate payments, etc. for you. Also, I assume you know that, depending on how many cruise cabins are booked, you will be eligible for extra perks and/or savings (although this is not exclusive to RCCL--most lines do this.). Depending on what you want and size of group, you can probably have a special party or reception, for example.
I think cruises are great choices for family reunions--there's something for everyone.