Worried & Need Advice

My siblings and I have abusive parents (emotionally, physically and sexually). We were placed in Social Welfare care until we were of age to leave. My mother has past away while my father continues to emotionally abused me and my little sister. My sister has two children of her own. My father has then gone to Social Welfare and told them that my sister abuses her own children, which I know that it's not true and I can't understand why Social Welfare would believe his story with his history of abuse. Since then, my sister and her partner have asked professionals in the community to help because she can't cope with his abuse anymore because she almost lost her children. They have asked doctors, social workers (not part of Social Welfare), counselors, and even the police. What shocked me the most was their response. "Oh, she needs to be on pills and locked up", for what, she didn't do anything wrong. So the best they could come up with was a trespass order, while my sister continues to be abused. I just don't know what to do because there is no one I can talk to, not even in the community. I just don't want to lose my sister or my nephews.