Wheelchair rentals are available at the front of both theme parks, as well as in the parking hub.
All queues (except Back to the Future and Kongfrontation) are fully wheelchair accessible. This means that your entire party (including your mother) will be able to stay together. Of course, this also means that you will be required to stand in line along with all other guests (sorry, no "bonus" front of line access for renting a wheelchair).
There are, however, a few Universal Express queues that are not wheelchair accessible. Therefore, should your party use Universal Express at these specific attractions - once your "reserved" time has arrived you will most likely be directed to a seperate entrance. But you will still have to wait for the time on your Express ticket.
In terms of accessibility on the rides themselves - you won't find a theme park anywhere that is more accessible for guests with disabilities