The best advice I got after having DD was:
If those who watch the baby will not or do not do as you say, then they do not get to watch the baby.
Of course this was from the pedi and it was over sleeping on the back and not giving a two week old straight water, etc.
You maybe being a bit hard on your MIL. But you are your child's mother and if you feel her care is not correct or is not right for your child that is really all that matters.