I want to adopt my niece but mother wants her back

This case would be a contested adoption. I just want to know if it possible to get custody of my niece. I have been raising my 7 year old niece for the past 4 years. I would like to adopt her but I know my sister wouldn't sign over her rights. she has not been supporting her financially and has not been living with us. I see my niece as my own daughter. Is this something I can possibly fight for in court?