MTpaperworks: What your sister needs is called a "Guest Assistance Card" or GAC. She does not need any note or documentation from her doctor. Here's what you do:
When you arrive at any of the parks, go to Guest Services/Relations. She should explain her foot problems and inability to stand in line for long periods. They give her a GAC. The GAC is a tool to let the CMs at the attractions know what sort of assistance is needed. In your sister's case, she may need to enter the ride via an alternate entrance to decrease the wait time.
You need to get a GAC in person at the parks (it is issued in the name of the person with a disability, so they need to be there to get one). The GAC is usually issued for up to 6 people (5 plus the person with a disability) and it is good for the length of your stay. The GAC you get at one park is good at ALL parks (except the water parks).
For shows, talk to any of the CMs on duty at the entrance and show them the GAC. They may be able to give you directions for how to get into the show without having your sister stand for a long period of time. With some shows/attractions, the CM may be able to let you enter/seat your party before everyone else comes in. This is NOT guaranteed, and some shows may not be able to accommodate every type of GAC. For live shows, with a GAC, usually the CM will direct you to seats in the front, but again that depends on the nature of the disability. Just follow the CM's instructions.