I agree with most of the above.
Sleeping arrangements - class Cs that length usually have a dedicated cabover bed and a dedicated queen bed. Add 2-4 bunk beds that may or may not be full size, and you are pushing it for sleeping 4 grown adults full-time. Add in additional people part-time and you've used your dinette and/or couch for sleeping too. That is going to be very crowded and confining.
Storage - full-time gear for 4 adults is going to take up a lot of space. With the bunk beds, you will lose valuable storage space. Add gear for the part-timers and you're going to be extremely packed.
GVWR - when you add water, 4 adults, all their full-time gear, you are going to be closing in on your numbers. Add the part-timers and their gear and you are looking at even tighter numbers.
Practicality - when your mom wants a mid-afternoon nap, what are the rest of you going to do? You will be stuck at the RV parked. What about the grandkids? Mom won't be able to "go to her room" and get away from the noise? What about when someone gets sick? You will be in very close quarters with a lot of people. In addition, you will all have to agree with everything - when you go shopping, sightseeing, staying at camp, etc. Because your vehicle is also your home, if the twins want to go shopping, you and mom have to go along.
Do you plan on towing a car? That will affect your GVWR, etc. It will give you more versatility as to splitting up for activities, etc.
If you are realy serious about this, I would consider 2 RVs. That puts 2 people in each full-time. If you do trailers instead of class Cs, then you can set up the trailers and use the tow vehicles to get around. That would allow some people to stay "home" and relax while others go sightseeing, shopping, etc. Of course it will double the cost for the most part - but I think the ability for each person to have more independence will be well worth it.