We did a big family trip 3 yrs ago (before we had our kids) and it was great. Our whole "core" family went - my parents, my sisters and our dh's, my grandmother, and my niece. One problem you might come into with planning is that noone else might want to plan, at least that is how my family was. Here is my advice:
1. STAY ONSITE - that way you can all go back to the hotel when you want or go to different parks. Gotta love Disney transportation!
2. Get walkie talkies - we could always find each other when we wanted them this way, and they were just plain fun
3. Dont be afraid to split up - we spent a lot of time apart and would meet for parades and things like that
4. Take lots of pictures - I have 2 that are priceless - one with me and my sisters, and one with me, dh, my grown sisters, and their dh's with Mickey
5. When someone starts being grumpy, head the other way - it is much better than fighting with them
6. Don't let anyone else get you down- I am an extreme morning person on vacation (never anytime else though) and no one besides dh (who is the same way) shared in my enthusiasm. I just ignored them and had a blast.
Most of all relax and enjoy the fact that you have a family and you can all be together. It can be really important to remember that when they start bugging you (trust me, I know!)
Kristy