Hey Dana, that's such a great present! Does your mom need another daughter?
Seriously though, up until just over a year ago I was living about an hour from Orlando, so I spent a lot of time at Disney World. I was 5'0" and 245 at the time, and I could fit on all the rides. The only one I had some issues with was Space Mountain. The turnstiles to get into the ride are seriously tight, and I had a tough time. Then you have to squish into the car and getting in and out of that thing (you sit on the floor of the car) was less than graceful. I fit, but it wasn't very comfortable.
I think your sister will have issues with some of the older rides, but honestly, Disney tends to cater to Americans' expanding waistlines, so chances are she'll be fine on the newer stuff especially. My favorite is the safari at Disney's Animal Kingdom and that's just an open bench seat in a truck, so you'll have no worries there at all.
Have fun, and try not to worry too much about it. You're all going to have an awesome time.