How can I provide my sister a safe environment?

I have a 13-year-old sister who is living with our mother. Our father passed away 9 years ago. My sister is being tortured by my mother, who is mentally unstable and tends to torture those around her. My sister attempted suicide due to the situation, and I had to rush her to the hospital. Despite police investigation, no action was taken. My relatives have kept her for some time, but my mother is creating trouble again and forcing my sister to return home. My sister has asked me to take her away. I want to provide her with a safe environment to study and live. How can I give my sister peace?