There were multiple times per year that my step Dad (and other adults) should have called the police on my mother for violently assaulting me, but they didn’t. Nor did they intervene and try to get her 5ft 11 18 stone ass off my 5ft 6 stone one, either.
Result? Insane amount of injuries, CPTSD, NC with 95% of my family for enabling her drunk, coked up behaviour.
He did call the police when I eventually retaliated, aged 16, by grabbing a wine bottle and smacking her over the head with it, because I was on the floor, about to black out from her strangling me 
I was “sent” to go and live with my Dad, a man she had alienated me from since I was 7, who’s court orders she had ignored, who I barely knew, and who she told a pack of lies about me to. Fortunately, he didn’t believe a word she said and he spent a fortune relocating us 250 miles away/on private therapy.