Custody(?) Advice Needed

Hi. Throwaway account for safety reasons, but in need of advice about possibly gaining custody of younger sister/what to do. There's too much backstory for one post but I'll try include as much as I can.

This whole situation is about two of my younger sisters, and my mother. One sister I will call Mia, the other I will call Daisy. Mia is 8 and Daisy is 14.

Since Mia was born she has been attacking, abusing and generally mistreating Daisy. Throwing things, kicking, slapping, punching, pulling hair etc. Mother is aware of this and this has all been going on pretty much since she was born.

She acts like she owns the world and that everything and everyone is hers and should serve her. She's the most abusive, manipulative, hypocritical and narcissistic person I've ever met and she's only 8.

At first we all thought it was just her playing with Daisy but as the years have went on its only gotten worse and continues to do so. I recently moved back in with Mia, Daisy & Mother and wasn't fully aware of this behaviour beforehand.

Mother is a single parent of myself and my three sisters (3rd isn't relevant to this story ATP.) She has severe mental issues and used drugs during all of our childhoods, up till Mia was born and occasionally afterwards.

Our family has always been dysfunctional AF but Mia has been a significant contributor to all of it.

She starts fights daily, attacking/hitting Daisy and then when Daisy fights back to defend herself, Mia runs crying to Mother and plays the victim, and Mother sides with Mia. Every time, no matter what happened.

It is ALWAYS Mia that starts attacking/harassing Daisy and then she tries to manipulate Mother into siding with her/punishing Daisy for defending herself. Most of the time this works and Mother brushes off Mia's behaviour, acting like she's just being a kid, even going so far as to say "She's just 8! Get over yourself!" to Daisy. And telling her not to hit her back, though it's self defence.

Mother has never been what I would call a good parent. She used to hit me, Daisy and my older sister. As well as yelling, screaming, starving us etc. And now Mia does this behaviour and she rewards, excuses and ignores it. If this continues someone is going to wind up hospitalised or dead.

I am moving early next year and want to take Daisy with me as she isn't safe here and Mother won't do anything to correct or punish Mia's behaviour.

I am 21, have enough money saved up and will be getting a job as soon as I land a rental. What can I do take her with me?

I haven't got a car so we can't just dip (Would need a ride or to catch the train), and if I did Mother would probably call the cops. If I ask for her consent to take Daisy she is most likely going to say no, as she thinks Mia is innocent, or acts like it anyways, even when she directly witnesses her behaviour.

Do I apply for custody? If I get DCJ involved will they just take Daisy? Do I just take her and risk Mother getting cops involved?