The answer is yes, you can use your mother's line to upgrade to a new phone. After that just switch her new phone to your number and she can have her old phone back. That's how I got my S4 upgrade. I use my fathers line for upgrade. I have unlimited data and under a family plan with four lines to play with.
Has anyone used Portable Wifi (Google Play)?
I put it on my Verizon S4 and it works just fine (not yet rooted either...).