If you look at the guidemaps, every ride will have a symbol on it. Some rides, like Small World, you can ride the ride without even getting out of the wheel chair. Others, you can take a wheel chair, but is you're in a
scooter, then you have to transfer to a wheel chair. Others, you can take the wheel chair to the ride, but then you have to be able to transfer onto the ride. (Kind of like switching from a wheel chair to a chair.)
The Cast Members can't physically assist the disabled. (I assume it's a liability thing. These people aren't certified Nurses, or anything.) But you are allowed to assist your Mom.
Sometimes, transferring simply means what I think of as a "stand and pivot", where my Mom stands up, rotates 90 degrees, then sits down in a different chair.
Other times, it's tougher for Mom. I can't take her on Pirates of the Caribbean or Splash Mountain, because she'd have to step down into the boat.
I assume from your comment that you're considering going to the park without a wheelchair, and getting one at the park.
One thing they've started doing that I think was a really Great Idea. At least at Animal Kingdom (and I assume they do this at the other parks. AK is just the one I've seen), when you park in the handicapped parking, they have a dozen or so wheel chairs sitting at the handicapped parking lot. These are for you to use, so you can get from the parking lot to the wheelchair rental counter.
One possible thing to watch for: At least the way things used to work, rental wheelchairs/scooters could only be used at one park. If you wanted to go to a second park, you had to turn in the wheel chair, go to the other park, and rent one there.
I've read that, if you save your receipt for the first rental, then they won't charge you for the second rental. But you still have to be able to get from Park1 to Park2, and there's no guarantee that Park2 will have any. (They do run out.)
I've read that they now have a solution for the "one park rental" problem, but I confess that I don't remember what it is. (We used to rent wheel chairs, years ago. But now Mom has her own, so we don't rent any more.)