To be able to create a user account of any type, you need to login to an existing user account which already has admin priveleges. There's no doubt such an account already exists but it will almost certainly be password-protected, and your mum is unlikely to have that password.
You can't log on to the default administrator account either because your mum's account doesn't have the necessary privileges to enable it (by default it's disabled & hidden from the log-in screen).
Therefore, your only option is to run factory recovery if the recovery partition is intact on the hard drive (that's assuming it had a recovery partition in the first place). Factory Recovery will reinstall Windows to it's "out of the box" state when the laptop was new. All prior accounts & data are deleted, Windows prompts to enter your name & this is used to set up a new user account which automatically has admin priveleges.
Post make and model & I'll look up the recovery procedure for it.