Here's a
link to the thread the OP posted on the disABILITIES Board with several answers.
If your sister is under 18, renting an
ecv is not an option. Those are only rented for use by people over 18.
As Gail posted, your doctor might have some advice about rides not to go on. Disney won't prevent you from going on any, so it's up to you. The park maps do have a warning icon that indicates which rides are not recomended for people with neck or back injuries - I know that doesn't apply to you, but it is an indication of which rides are sort of "rough". Also, you can stop in Guest Se4rvices at any of the parks (or possibly at a WDW resort, although we've found they are often out of them) and get a Guidebook for Guests with Disabilities. There is one for each park. They don't go into a lot of depth, but they do give an idea about whether a ride is a gentle boat ride or a turbulent thrill ride.
The rides I would be especially cautious with would be ones where you have to step down into a boat or over a large threshold to board.
Pirates has about a 6 to 8 inch step down to the seat of the boat and then you need to make another long step (about 12 inches to get your feet from the seat to the foot area). That is hard with only one good foot. The Maelstrom boat in Norway at Epcot is similar.
Small World and the rdie in Mexico have similar boats, but they also have a ramped wheelchair boat. People can stay in the wheelchair and be driven onto the boat or you can ask to use the ramp so that you can enter and exit easier if you are not staying in the wheelchair.
Splash Mountain is another boat with 2 steps to board, but the high seat backs make it kind of tight.
Peter Pan is does not have a difficult ride car to get into, but the moving walkway on Peter Pan can't be stopped for loading, so it is hard to get on and off unless you are quick. (I put a link in my response on the disABILTIES Board about ride loading).