My friend is picking on my little sister what should I do?

My sister is five years old and my friend is fifteen the other day she came over we where just doing what any regular teenager does chilling on the couch watching TV. And then my friend said she will be right back after a while I was wondering where she was so I just decided to wait and then I heard my little sister crying I went in to her room and she was bleeding and her doll house was on the ground and her barbies where al recked I hugged and said what happened and she said my friend cut her with her nails and threw her doll house on the ground and was ripping the dolls apart. My Mom told my friend to get out I felt harsh for kicking her out but at the same time I was pissed about what she did to my sister.