What Do You Do To A Father Who Batters And Humiliates Your Mum In Front Of You?

I'm Frederick (pseudonym) i come from a family where raising your hands against your parent is a taboo, my mum has been with my dad for 21 years, she's a second wife, the first wife is living elsewhere, they got divorced before my mum came, now my dad disrespect my mum everytime, he barks at her for nothing, even things that shouldn't cause trouble he'll pick offense, he beats her in our presence, back then when I was young and helpless I'll just watch and cry but now I'm grown, i can beat him up anytime now because I told him the last time that if he touches my mum again I'll kill him, i regret telling him that but i was enraged, this poor woman endured this for 21yrs, currently I'm not at home, i got a call from my kid sis telling me he has started again, since my last warning he was quiet, now he's back to his old self, the worst thing is, he has other children, he'll always call my siblings and tell them bad things about me and mum, they don't talk to her or me(my elder bro and sis) dont talk to me or mum, I'm the second child from mum and the ninth child from dad, he's always battering mum and causing a rift between me and my siblings.

He pays thier school fees including my old bro who's almost thirty and my immediate younger sis but he refused to pay mine, the only siblings that talk to me is the last born and the youngest girl, the rest sided my dad

Please what do i do, i dont want to be someone who beat up my dad