Hi Sarah,
Welcome to the Disney Parks Moms Panel with your question. I think you may qualify as the "best aunt ever" for considering to bring your nieces along with your family to Walt Disney World Resort! I'm not sure of your exact party size, so I will offer some general recommendations, however feel free to send me another question stating the exact size of your party.
Traveling with a large party can be super sized fun, however it does require a little more planning as you will have to make sure everyone's needs are met, and that you all "fit" comfortably in your accommodations. Seeing how you are used to staying in Deluxe Accommodations, a Villa would be the best fit, however they can be quite expensive. Depending on the size of your party, most Deluxe Resorts offer Suites which may be a more cost effective way to meet your family's needs. Disney's Wilderness Lodge Resort offers a Deluxe Club Level Room with Concierge Room that sleeps up to six people as well. This room is spacious, yet does not have the price tag of a Villa or even a Suite. Connecting rooms may also be an option for your family.
If you are willing to look outside of the Deluxe Resort Category, I would suggest looking at the Cabins at Disney's Fort Wilderness Resort. Although the accommodations are slightly more rustic, there is plenty of space for a larger party. My final suggestion would be the suites at Disney's Art of Animation Resort. We stayed at this Resort over Christmas and although it does not have the high end fixtures, there is plenty of space and the pool is fantastic!
I hope my answer helps Sarah. Please let me know if I can be of further assistance as you plan your Walt Disney World Resort Vacation.
Stephanie