PLease Help Me About My NIECE's CUSTODY

I want to know how I can have my 9 month's old niece's full custody since her mother, who happened to be my second sister, always neglects her. My second sister often leaves my niece to my third sister and my care because she always goes out with her friends every day and  night. I and my mother supports the child's finances because my second sister doesn't have a job. My niece's father is a 16 year old child who is a high school student therefore cannot support the child. My second sister is turning 21 years old this 28th of May. Yesterday, when I was at work, my second sister physically abused my daughter leaving my daughter with a swollen face. My third sister who was taking care of my daughter at that time got mad and told my second sister to get out of the house. What my second sister did was leave the house with her daughter and until now she is not yet home. I am so worried about my niece since her mother doesn't have a money or a job. I want to know what I can do to help my niece.