How would you split up the family? I am inclined to say if the aunt is not cooperating give her the choices that she could choose for her family and you do your own thing and just meet up at the parks.
If that isn't possible then break it down in writing. Two rooms at SSR would be $165.00 a night and total cost would be XXX. Three rooms at a moderate resort would be XXX and total cost would be XXX.
Also provide her with the benefits of staying at SSR instead of a moderate or value resort. IE: Full size kitchen, separate rooms, extra bathrooms, etc.
Another idea is maybe staying at two of the family suites. Therefore you will have two kitchnettes, four bathrooms and you don't get charge extra for the adults.
I wish you luck. It is hard planning a vacation with multiple family members. Everyone needs to be on the same page and how the expense will be distributed.