A few years ago, I was on crutches for 6 weeks. I could barely make it a block without feeling miserable. No way would I attempt to take anyone on crutches to WDW. It would be awful for everyone involved. Not only will your niece end up sore and exhausted, but you will feel the stress wondering if she's in pain, if you're moving too fast for her, etc. Finally, crutches can be a safety issue in a crowd. It's hard enough to maneuver through crowds on two feet. She could be easily tripped with so many people around.
Here's what I would tell your sister. "DN is welcome to come with us only if she uses a wheelchair and only if you pay for it. We will not take her only with her crutches. If you refuse to pay for a wheelchair, then she will have to wait until next year to come with us." Period. No negotiation. It's for your niece's comfort and safety.