Hmmm well vet bills for rats can be very expensive so try getting a job and pay for them yourselves. What kind of shavings do your grandparents use? Pine and cedar are dangerous to small animals especially rats.

Show her you are responsible and pay for as much as possible yourself and research thoroughly on their care.

Let your mom read this: Rats are not dirty or disease carrying animals. They are cleaner then cats, just as friendly as dogs and they don't bark. They are great family pets and are more social then other small animals. In my opinion they are the number one pet to have. I have had all sorts of fish, hamsters, gerbils, mice, dogs and cats aswell as rats. Rats are cheaper and less work then cats and dogs but friendlier then the others.