5 years is a long time. While you are a party of 12 right now, in 5 years that number might grow...
I agree with the advice PP gave about opening a saving account and use it for your Disney vacation dollars saving.
Start a notebook, so that you can keep up with suggestions, or things that you might want to consider along the way.
Depending on nights have you thought about AP's? this is another factor that you might want to consider.
How many nights is important? the more nights the less. Peak times or around holidays, or a new land or attraction opening can add to the cost.
On-site or Off-site resorts there are many perks to both, Price or amount of space or a combo of these two. I think is normally where most people start when planning. While being together is wonderful, each family will want and need some private time. So this will also go into planning and budgeting.
Family suites might be a good fit, this way everyone can spread out some. You know when the kids get to a certain age personal space starts to play into things. Our friends ( party of 20 for a family gathering) decided on Family suites at AOA, the original plan was POFQ but after realizing that everyone would be together for 2 weeks ( 8 nights at WDW) her DH thought it would work out better for everyone. Another reason they went with family suite at AOA, was 2 bathrooms, plus there is a door that can be shut so mom and dad have some privacy. As well as room for the strollers, pack in play, luggage.
Getting into dinning here is where a lot of people disagree on the best way to go. With a big group, I personally would use the dinning plan, DH and I used the DDXDP when we took the kids and grandkids, this way we did not worry about who got what, less stress for us. Before committing I would take into consideration how does your family normally eat? big eaters, picky eater, dietary issues, 3 meals a day, grazers, whatever. Also the age of everyone going should be considered. All of a sudden that kids who barely would finish his or her chicken nuggets, is now all of a sudden eating a 2 double cheeseburger, large fry, and drink, and looking around the table to see what everyone else might not be eating.
Once things get closer like within a year of going you could start giving Disney gift card as part of Birthdays gifts, tuck some in a Easter basket, or Christmas gifts so that everyone has some spending money of their own.
Have fun with the planning..