I wouldn't take your mother - where would she sleep? Plus, my own mother and my own mother-in-law get along "fine." But not "vacation-for-a-week" fine. They'd drive each other batty - and I'd have to commit ritual suicide before the week was out.
Sit down with your husband and figure some stuff out. Sounds like he envisions himself sitting around the villas with them and your niece, leaving you on your own (i.e. you can take your mom so you have something to do). If that is fine with you, great. I'd be setting my hubby down for the come to Jesus talk - WE do these things - a nice dinner, time in the parks. WE aren't going to be bringing your neice down to sit by the pool. If your parents want to sleep late and hang in the Villas, great - we will bring cell phones so they can reach us if they want to hook up. I'M not thrilled with them coming to start with and if you decide spending time with them is more important than OUR vacation, I'll get quite cranky.
That is actually a pretty strident version of the deal I had with my Mother In Law. We are going to do these things. I'd recommend you do these things with us. But you are free to spend as much or as little time with as as you'd like. In our case, we spent the first two days running my MIL and her sister ragged - we wanted to get them the lay of the land. They spent the next four meeting up only for dinner - we went to the waterpark, we rode rides - they poked through Epcot. We spent the last day together again doing the kids favorite MK stuff.
And I did "express my concerns" to my husband before inviting them. That in the interest of being a good hostess I'd have a lousy vacation. That our kids were only going to be this age once - and we'd bought
DVC so that we could see Disney through their eyes at each age - and not the "we've been in this shop for an hour mom! I'm bored!" experience. He understands what Disney means to me and we are both very aware of my (and by extention his) vacation style at Disney - early mornings, quick moves between attractions. Not quite commando, but not a lot of room for "this is vacation, I'm going to sleep in every morning, and if I spend an hour looking at doo dads in a store, that is a good way to spend my time." And we are both very aware that is mother is a "sleep late, move slow, like to shop" sort of person.
(However, all of us, MIL, DH and self, like food - so we centered the whole vacation around where we were meeting for dinner - sharing the part everyone was very content with).
All in all, the vacation was fantastic. Everyone had a really good time (I did push too hard the first two days). My mother in law was great - I did a great job planning to our incompatibilities, and she was a real trooper when she needed to be. And her sister - I'd take her anywhere again in a heartbeat! Gracious. Wonderful traveling companion. Helpful. Not obtrusive. And the best thank you note I ever received.
The only other thing I wish I'd done is we were so busy, we never took a night to let my MIL babysit so we could go out. It wasn't important to me, so I didn't schedule it. But SHE saw it as a way to pay us back in some form. I didn't realize that until much later and I should have been more aware of it.