Rebecca,
You are so sweet to think of giving the gift of time to your mom and brother. I have been the happy uncle on a trip or two in the past, and totally know what you are going for.
I would suggest that you offer to take your niece and nephew for an evening. That way your brother and mother can have a nice dinner, and maybe explore a park together.
Meanwhile, you take your niece and nephew to whatever park or destination they want to visit. But perhaps start it off with a fun, age appropriate meal? I would take them to the Whispering Canyon Cafe where they can explore the Wilderness Lodge a bit, have a great meal, then move on to the evening's destination.
Now the Magic Kingdom is an obvious choice. But really any park could be a good one. Let them decide.
But what if they want something OTHER than a park? In November the water parks typically close at 5, so that is out. Downtown Disney could be fun, especially if you could budget some money for them. Start the evening off by giving them an envelope with some cash or gift card to buy something.
But what else could you do? Instead of going at night, you could take the kids on a tour experience during the day, like the
Wild Africa Trek in the Animal Kingdom? That might seem less "kiddie" to them.
So parks at night, Downtown Disney, or maybe a tour? Do any of these seem to fit the bill? If not, write me back with some thoughts on what they like and I can offer more directed suggestions?
Have a magical vacation!