We have a friend who watches our son (2 1/2 yrs old) all the time, and he is very lenient and spoiling. However, I notice that my son does respond differently to discipline based on who's giving it (I make the most effort at it, and he listens to me more than to my wife or our friend). So I wouldn't worry too much about Grandma spoiling your daughter as long as your daughter still listens to you and your husband. (Also, consider yourself lucky to have family around -- my mother lives 1200 miles away, and my mother-in-law lives 12,000 miles away!)
On the other hand, if your mother doing things that risk your daughter's health is an ongoing problem, then you definitely should sit down with her and lay down the law. If it happened once or twice, then hopefully you can just mention it and it won't happen again. But if you're worrying about your kid every Saturday, then you need to make clear to your mother that she needs to be more considerate of your daughter's health. Better that she should feel offended then that your child be sick all the time.
On the plus side, the fact that your mother gets very upset at the prospect of not having Saturdays with your daughter, that could be enough motivation for her to be more careful when it comes to watching out for your child's health.
Good luck!