How do I tell my MIL that my 2 month old son is too young to be babysat?

My MIL is dying to babysit my 2 month old son but every time my in laws are around my son screams bloody murder (he does not do this with other people - just them). I'm afraid for them to babysit because it might traumatize him. Also, I am uncomfortable with her taking care of him because I have witnessed her verbally abuse and swear at her other 4 year old granddaughter and I do not want my son subjected to this, although I'm sure this will be more of a problem when he gets a little bit older. What do I do?