B.M.
Dear J.,
First let me say that this is one area where I would fully trust your woman's intuition. Any time I have felt that way about someone, there has been a reason for it. Your child's safety is first. I understand that you don't want to offend your mother-in-law. But if something precious is taken from your son, it cannot be replaced. I'm not sure how to help your husband understand. Have you ever felt funny about someone who ended up making a pass at you, or some such thing that you can use as an example? Is it possible to have her spend a weekend here and there with you two - maybe to watch the baby and give you two a chance to have some downtime - or when you want her to babysit, could one of you drive out to pick her up, then take her back home after?
This is a difficult situation. Good luck with whatever you decide to do . . .
B.