The buses that are a problem are the older buses that have lifts. Some very large
ECVs are too long to fit on the lift. The newer buses have ramps and are not a problem. The ECVs that are too long are ones similar to the ones you can rent in the parks. The ones you rent from the off-site rental places are much smaller. If you have any concerns, just make sure the company knows that you plan to travel on the bus with it.
That said, I think your sister would be better off having someone push her in a manual wheelchair.
She will give up some independence, but it will be less tiring. You have to be alert and always watching while driving an
ECV. Plus, you have to keep your hands in a fairly consistent position to drive. If she is weak and in pain, staying in position and staying alert is going to be even more tiring. Plus, if she is on painkillers, that will make her more tired and less alert.
My DD drives a power wheelchair. She is an excellent driver, but after an hour or so of driving in really crowded conditions, she's tired and wants someone else to take over the controls for her for a while.
I'd also suggest you consider renting a car or van and drive to the parks (except for MK, where the bus lets you off right by the turnstiles). That way, if she is needing to leave, you can leave pretty quickly and not have to worry about when the next bus is coming.
If she's in the park and needs a short rest, she can go to First Aid and lie down. The medical CMs there are very nice.