I've had the same thing for years with my IL's. I was brave once and told MIL not to feed something to my DD while she babysat, and she ignored me. DH only found out some hurtful stuff in the last few months. Our problem is if i say something MIL ignores me and does whatever (I've had the MIL grappling onto DS while I'm trying to take him back too) and if DH does it, then she whinges to SIL that (and even said it to DH face once) that it isnt him talking, obviously meaning me putting words in his mouth. I am at the point were I am going to blow up one day very soon, and I will be telling certain people they are not welcome unless they are fully respectful. they dont have to agree or like, but they are our children to parent, not theirs.
I would say or do something now if possible. You wouldn't want this to continue on and on and on, as they just become more confident that they are right and you are wrong.
What Kim said about chatting with your DH so you are on the same page is a great idea. If you do have a showdown one day, you want to know that he agrees with whatever you are saying and vice versa for him too.
hugs