My 7 year old sister is suicidal.

Sorry its long🙉
To begin, my step father (her bio dad) is a narcissistic a-hole (he literally says things like "im smart and youre stupid", "im bigger and better than you and i will always win". Just like the film Matilda!!) and my mother cheers him on meanwhile telling us to "drop dead" and asks us things like "wouldnt you love it if i just went and killed myself??". 
They have 3 daughters together and i'm the eldest. I am 22, the next one is 17, then 7 and 2. 
I have complex PTSD as i have suffered quite a lot of trauma & sexual abuse from my bio dad... so I am very spiritual and i am an empath and i KNOW something isnt right here, i can see it a mile off. 
My 17yo sister is gods gift to earth, she almost died twice, her twin died and she was their first born so she doesnt know or care to know the impact of this abuse. Their second child (7yo) kim, is my blue eye i raised her like she was my own and ever since i left home 2 years ago she has completely changed and her behaviour became out of hand, she began attacking her grandparents, screaming at everyone, slamming doors and yelling crazy things, she literally flipped like a switch. She isnt my little happy content baby girl anymore. They treat her somewhat similar to how i was treated. They constantly scream at her, ridicule her, call her nasty names and force her to do things for them because they cant be assed meanwhile calling her a stupid d-head. The other day i recieve whats app messages from her that really concerned me. My mother and her partner NEVER admit to anything. They have denied everything they ever put me through, every punch, every 10 minute smacking session, every text to say they were waiting with a belt at home, i remember it ALL and they honestly tell people i live in a parallel universe. Last year i went into psychosis of trauma and had to be admitted for a month because of horrific flashbacks. So this is forever held against me and they call me crazy. The stigma of mental health almost ruined my life. May last year, my mother became estranged, completely detached herself and i dont see or hear from her. Its like a wild goose chase to get in contact regarding the kids. She doesnt give one single f. She tried to tell me i dont care about Kim, of course i flipped out, i gave her a deep, in depth message in return of how much that child means to me, what i have done for her, what i would do, and every safety measure i have installed into her. Talking to my mother I obviously used different terminology and told her the amount of kidnappers and pedophiles out there these days, i HAD to keep her safe. My mother sees the word "pedophile", somehow linked it to what i was going through BECAUSE of a pedophile, and ran to a local women centre telling them i had been teaching the child about pedophilia who then contacted social services and my contact with my sisters had ended. She completely ruined my life, She vindictively took the only thing keeping me alive, for no good valid reason. So my heart is in my mouth panicking about wtf im to do, If i go to my mum with these messages she will probably somehow twist it and try to accuse me and i honestly cant be losing my sisters again theyre all i have. Here are the messages. What would you do?????? I feel so stuck and i know i need to tread carefully. I cant not do anything im seriously concerned.