Should I allow my MIL to be alone with my son?

Oh sweetie, you definitely handled it better than I would have! Honestly, never letting her see him again isn't too realistic, however, you need to make sure that she understands YOU are the MOTHER and that what YOU want will not only be respected - but followed through. That is inconsiderate and I can't believe someone can be that ill-informed and simply stupid when it comes to milk! My son is 9 1/2 months and has never stayed the night anywhere else from me. No, wouldn't say you are overreacting. I would have done much MUCH more than what you did :)