I Need Custody Of My Sister, Mom Refuses And Wants Her On The Street.

Im in need or legal advice for my lil sis (16) who i have in my home currently. My mother and her have not gotten along since i went low contact 4 yrs ago a year ago when i got married. the two of them would get into fist fights and finally my mother started to send my sister to live anywhere but with me. When i tried to pick her up she called the cops and said i was kidnapping her daughter…. A year has passed since then and my sister has once more been kicked out and my mother has stated that she is not to come home or come to me but wants her on the street. I want custody of her and just want to make sure i have a strong case. I have no idea what im doing or if i can afford this but im all in now. Im willing to share as much info as needed to make sure there is no confusion.