Can I have full custody?

My sisters 8, her dad (my dad) is nearly 60, she’s been taken off her mum (alcoholic) and been living between her aunty and dad who lives a couple hours away from each other.
The aunty is in her late 40s with no experience of children which means she can’t teach her morals, and the whole switching guardians will mess her routine up.
She was living with her nanny who’s too unfit to look after her obviously and has been inbetween the 2 for the past 2 weeks. She’s no longer in school and I’ve just found out her mum drank heavily while pregnant with her.
I’m 19 F she’s 9 F, I’m trying to move out and continue my course in college, I only want her to be safe and brought up in a happy and safe environment so she doesn’t go down a bad path with all this moving about and not being in school.
We never been extremely close because she’s my half sister and lived with her crazy mum until now, she’s the only sister I have, I live with my mum and have a large loving family with lots of cousins and aunties and uncles who would show so much love and support. What’s the chances that I could request full custody?
And the fact that her dad works in an airport and works nights isn’t ideal for a 9yr old