You say one of the few situations she would need to drive you somewhere would be in an emergency -- in an emergency situation, regardless of "law" in THIS situation, I would have her drive me. If she gets pulled over and the cop sees she's with her mother and there is clearly an emergency, I really hope the cop won't be a you-know-what and give her and/or you a ticket.

It's all circumstantial... unfortunately you can't make laws that bend to circumstance