I'm an MB. I am of mixed minds (based on what you've laid out here).
I would be annoyed that the nanny you hired, who accepted a position knowing that these would be the terms once your maternity leave ended, is no longer willing or able to do the job she accepted. It would be perfectly reasonable for you to simply find another nanny.
OTOH, it is hard to start over and if you ended up with a situation where you actually had some built-in back-up for illness (if the nanny is sick her mother can fill in and vice-versa) then perhaps this would be an unconventional but great solution for you in the long run. It's also the kind of accommodation that your nanny might see as a major benefit so it could be a win win.
Either way, if you're going to consider doing it, you have every right to interview her mother just as you would a new nanny. Explore her abilities, energy level, experience, your comfort level with her etc... Make the requirements of the position (hours, activities, driving, etc...) all fully clear and non-negotiable. If you're persuaded that she is as capable and attractive a candidate as a new nanny - great. If not - then you go back to starting over.
Good luck.