I'm not a mother, but I am an "auntie". If my nieces' and nephew's parents asked me not to no certain things I will not do those things. i am not their mother. end of story. Your sil has over stepped her bounds. I'm sorry, but pressing a childs hair who is not your own, when they mother obviously does not, is wrong on so many levels.

I have known a few woman like your SIL. They wanted the family but never got it, so like to have "surrogates". They can be control and go over parents heads to feel like a mother. I think you should go to your husband first. if nothing changes, go to her.