If you have a look at the FAQ thread at the top of the disABILITIES board (this one) you'll find a list of wheelchair rental companies in the Disney area. It might be better for you to rent from an off site company (as opposed to picking one up in the parks each day) as she'll have the chair for the whole trip, the off site rentals tend to be cheaper when renting for more than a couple of days and they're also usually lighter (the WDW ones are the wheelchair equivalent to a tank... minus the gun.... hmmm, that might be a useful addition though

). You should also be able to rent one with crutch holders, so she can still get out and hop around when she wants to. If you go for in-park rentals, last time I checked it was $10 for one day or $8 a day if you buy a week's worth in one go. If you plan to park-hop, save the receipt and you get can get a wheelchair at any other park you visit for the rest of the day without having to pay again.
It's very unlikely that she'll be able to push herself all the way around the park, so you'll probably want to pass her between yourself and your mother, taking turns pushing the chair. Disney's a fairly easy place to wheel around, and everything's nicely accessible, but it is BIG.
Also, don't feel that just because you have the chair you
have to use it all the time. Some people feel uncomfortable being seen getting in and out of a wheelchair, but if she wants to walk for a bit, why not? Some people like to use the chair to get between areas or lands in the park, then leave it in the stroller parking area, walk around for a bit, then get back in to trundle on again (did that make sense?). The stores often get crowded, so she might feel happier going around them on crutches, rather than in the chair. Just play it by ear and do what you need to do.
For a few little tips for a first-time wheeler, K started this thread, which you might find handy
http://www.disboards.com/showthread.php?t=1543998 . Also, the FAQ has information on buses, rides and general access with a wheelchair.
If you have any more questions, feel free to post them up and we'll do our best to answer them. Good luck and have a great trip!
