What is wrong with my sister?

I have a three-year-old sister. Well, my dad and step-mother are divorce, and it is my dad's weekend to get the kids. 5 days ago, Hope (my ex-stepmom)Dad and said that Isabelle (my sister) was sstuttering She claimed that it was perfectly normal. Well, when Dad was giving Isabelle a bath, he notice a scabbed gash on her head. She will not let him touch it. All she would say was, "Mommy did it." My dad called our Aunt Shirley and asked her to come look at it. Again all she would say was, "Mommy did it." So, tomorrow when she wakes up I am going to see her to see if she will talk to me about tit. We have always had a very STRONG sisterly bond. Is there any advice someone can give me? I am deeply worried about her.