I have a 6-week-old right now, and I can hardly make it to the mall or the grocery store! I have actually thought about how hard it would be to be at WDW right now. Your niece might be okay if there will be a lot of people to help out, but I still think it would be hard. I had an easy pregnancy and an easy, normal, natural birth, and I am still just getting my stamina back. I can't imagine trying to walk around WDW. Plus the baby has to eat very often which means lots of stopping for feedings and diaper changes. These things are easy at home or on short outings, but I don't feel up to any theme parks just yet.
Oh, and I have a pretty easy-going baby, too; she is just normal for a 6-week-old. If your niece's baby is colicky, it would be even harder.
Now maybe your niece would feel differently from me, but this is just the opinion of someone in the sleep-deprived thick of it!