Are you still within the Setup Wizard, or did you already get past that? After you did the factory reset, did it ask you to enter the Google account and password that was previously on the phone prior to the reset? If so, then you have to enter your mother's Google information, since it's part of Google's factory reset protection feature. In order to get her account completely off the phone, you have to remove her account in the Settings>Accounts menu, then do the factory reset.