Step-mom misbehaves with my sisters and s them brutaly?

I am 15 years and my two younger sisters are 12 and 10 years old. My step mom used to beat my sisters.some times she s them. She alwayas abuse sisters. 2 days ago i was out of house my 12 year old sister complained her for not properley feeding them .step -mom became furious and she ordered her to strip otherwise she would complain to father.my sister was compeled and she removed her clothes.step mom beaten her and ed her brutally with comb.after she ed on her with comb without listening her. When i came back my sister told this to me .my father does not listen our complains against her.however i told him about that but he tooked no actions.stepmom behaves like a dirt with my younger sisters.how could i improve the situation and punish her for her misbehaves ?