My mum has recently lost custody of my little sister age 9. she now lives with her dad, but he is a horrible man, and used to treat me and my brother really bad. I'm nearly 20 year old, and I really want to go for custody of her, what would my chances be of getting her. Please someone can you give me some help and advice,

Custody arrangements between parents set up by a court often becomes difficult. You say that "your mom lost custody". Were your local welfare social workers involved? If they were then can you speak to him/her about you getting custody and your reasons?
You would have to show that you have the financial means and ability to care for her.
Good luck

You would have to prove that he is unfit and a danger and that you emotionally and financially can provide for her.

It's going to be difficult at your age.