Opinions please???

Hi mamas, I’m seeking advice not as a mother but as a concerned sister. My eldest brother(24) got into a physical altercation with my little sister(15) this past weekend. This is not the first time he has hit her and it won’t be the last. He has always been violent and cannot control his emotions. However we do not have a father and my mother seeks male validation and is a narcissist. So therefore she protects, lies, and will basically do anything in her power to cover up and defend her son. Leaving my little sis to fend for herself as I am no longer living in the home. I’m not sure who to call? Or what to do? I cannot let this man continue to hurt children and women. He also has a childcare degree, so you should be fearful that your child(ren) could be next!