Just curious, I know I'm late to the party, but there's no 2 factor ID, Google calls it 2-Step, set on either account is there? Not knowing which account it's asking for, which really should be your mother's since it was last setup using that account, you'll want it to be turned off for your attempts.
2-Step can be difficult to navigate when trying to set up a device that was factory reset using the button method vs the menu method. You may have to disable it for brief time while logging into and setting up the device.
Barring this you may have to contact Samsung for assistance. I doubt they'll give you a solution but perhaps the device can be returned to them for factory resetting to it's true default and you'll be able to start fresh. If there was lay-persons way of bypassing this security feature it wouldn't be much of a security feature. Barring the advice given here Samsung may be your only option.
Best wishes.