the trick with allergies, is that people are rarely allergic to the fur. its the actual dander, saliva, skin that causes the allergies. maybe take your mom to a pet store or friends house that has rats. hang out there, handle them for half an hour and see what happens. two rats are better than one but if she's allergic i'd hate to see you have to rehome them. I have bad allergies too, and I find experimenting before buying is the safest way of doing things. that way if you are allergic you're not stuck with the allergen in your home.
"If you can't change your fate, change your attitude." - Amy Tan