Can I Fight For Custody Of My Sister?

I am a 22 year old male with a part-time job. I live with my girlfriend and a roommate, who both have full-time jobs. My sister and I are technical orphans in the sense that our biological father has left us and our biological mother had deceased. My younger, 16 year old sister is now in the care of my mother's parents. My father's parents are unable to take custody of her. My step father is unable to take custody of her. I am the only remaining close family relative that my sister would like to live with. She does not like living with my mother's parents. She often feels misunderstood, neglected, and downright hated. We have had a rather dramatic history with the grandparents, and all of us had voiced our disposition on living with them. My sister has voiced her preference to live with me. My main concerns are that collectively, our economic situation is less than that of the grandparents. I rent the house I live in, and I have roommates. I believe those are the main obstacles.