Custody of Younger Sister

Hi, im wondering if its possible for me to gain custody of my younger teenage sister. I'm 23 years old and have a four year old of my own. Her father didnt want to be involved and signed away his paternal rights. I'm now engaged to a man who has been in our lives for a year and has two of his own children i intend to adopt. My 16 year old sister has lived with me for 2 years, since my parents decided they didnt want her to live with them anymore and moved from California, where we live, to New York for my father's work. She is doing well in school, is extremely sociable and an important part of mine and my fiancee's family. Our kids love her and she wants to stay with me. My parents recently came back into our lives after no contact for 2 years and have decided she should go and live with them now and its not what either of us want. Growing up my parents were never around properly for either of us and from when i was about 16, and had my license, they left us alone for days on end and i was looking after my younger sister.

Can they make her live with them or can i get custody or guardianship? i dont want my younger sister to have to be taken away and she doesnt want to be either.