We are so happy that your mother will be visiting us. Even though Jerusalem is built on hills, there are many sites that are wheelchair accessible.
For more information on possible itineraries, check out: http://goisrael.com/Tourism_Eng/Tourist+Information/Suggested+Itineraries/Itineraries+for+People+with+Mobility+Challenges.htm
Yad Sarah is a large organization that loans out a wide variety of medical equipment, including wheelchairs. You can also ask the concierge at your hotel for assistance regarding a wheelchair, as many of the large hotels have a few wheelchairs on hand and might be able to lend you one.
Have a great trip, we're waiting to welcome you.