PORS would not work as that would only allow 5 in a room not the 6 OP is looking for.
Hard choices, the BC is a beautiful hotel, but the cost for 2 rooms does get pricey. The villas for that time of year are alos pricey because you would need a 2 bedroom.
Best options are (if you have starwood points) potentially is the Swan or Dolphin if you can get a deal there, or the Fort Wilderness Cabins or 2 rooms at either the moderates or the value.
If pools are important (and with 3 boys I am betting they are ) the moderates all have slides in the feature pools while the values do not. The CBR have some neat areas for kids and the PO resorts are close to DTD. Nice boat ride and some nice pools for the kids.
It is very nice of you to want to include your niece. Now you just have to decide if moving from the Deluxe resort is a change to your family's vacation plan that you want to make.
Good luck!