I would contact Disney by phone and asked the best way to do this? Did you mum make the reservation in her name? If so, that could be the problem, and if they can't fix this, she will have to give you her account info so you could make changes and do fast passes. I would think they could fix it.
Only one person can make these, so linking it to your account isn't going to help unless they move it from her name to yours.