We stayed in a treehouse in March. Each Treehouse has a parking space right out front. Since your MIL has a bad knee you may want to consider a handicap villa. We had a handicap villa (we did not request it). These villas have a ramp leading up to the front door instead of steps and a handicap accessible master bath (no whirlpool tub - a bit of a bummer for us).
There is a bus that makes a loop from the Treehouse villas to SSR. It stops at all of the SSR stops. You will have to take this bus to SSR to get onto any of the park busses (or use your own car
)We had heard a lot of negative talk about the transportation but found it to be quite easy to get around. We were in Treehouse villa #7006. As I said this was a handicap villa AND it was right near the bus stop (the Treehouse villa loop bus). It was also right on the water. GREAT location!
We LOVED the Treehouse and plan on staying there again. It is an awesome place to stay with extended family. There is lots of room and the Treehouses are in just a beautiful location.
If you would like to see our photos of the Treehouse villa just send me a PM and I will send you a link.