Talk to your husband first, if that doesn’t work give it back to her.
She’s old enough to dish it out, now see if she’s woman enough to take it. If hubby doesn’t like it, let him have it as well. I would advise getting a NEW baby sitter as well. MIL be dammed! Hubby doesn’t like it, leave his sorry ass for not defending you and disrespecting you in front of their child from the MIL.
Be direct. “What was your intent by saying that? How did you want me to feel by making that comment?”
You’re not the asshole in the situation, she is— and she should know it’s recognized. 
especially in front of your child