From the Desert Inn, I'm pretty sure you can walk to the back entrance of the Timon Parking Lot and then catch a tram to the front entrance. We did this a few years ago and I'm not sure if it has changed. Your other option would be to park at the Mickey & Friends Parking Structure and take a tram to the entrance. If you park there, you might be able to find a handicapped space closer to the trams to reduce walking. I think your best bet would be to ask at the front desk of the Desert Inn. They probably answer questions all the time about walking to the park entrance and could direct you to the shortest route.
Is your sister planning on renting a scooter when she gets into the park? With a broken foot, that would probably be the best way to go.