However, if this resurfaces I think you can call Specials Needs at Disney direct and they should be able to give you the ins and outs of what can be done or not. Your aunt can do the same.
I agree with the other posters that a doctor's note doesn't get you front of the line access. If it did I would be first to do it.
Also, I think it's terrible the cousins won't push a wheelchair for her. I'm in my late fifties with bad knees and my overweight and I do it for my mom who is 88 and with Alzheimer's. I don't have a second person to help push a wheelchair, either. I'm not looking for any medals for doing it, either. I'm just sharing my story which I think is relevant, here. I did it for 9 days at Disney World last year. I'm going to be doing it for 14 days this year. I'm planning on it for Busch Gardens in Tampa, too; it's a much harder terrain there to push a wheelchair. I didn't win any races pushing a wheelchair, but we survived. I haven't seen my mom happier in a while; it was so worth it for me and her. We alternated between eating, rides, exhibits, live entertainment, shows, parades, fireworks, etc. By doing it this way I had a chance to rest and then move on.
Finally, your aunt can practice for free with a
scooter at stores like Wal-Mart and such. She can do this in her own backyard. If she wants to venture out she will need to learn a scooter and such. It sounds especially important since the cousins have such a poor attitude in helping her. It's likely not to get easier for your aunt; so, she needs to get adjusted to a new way of life. It's especially important to do it while she has full brain capacity. With dementia, alzheimer's, etc which can be around the corner (God bless that it doesn't happen), it doesn't get better to learn a scooter and such later on.