My question involves a child custody case from the State of: TN

My husband was previously married to a woman who had a 3 year old daughter by some one else. In time He had a daughter with her. Complications have came up to where we feel the girls would be better and safer with us. So we are wanting to get custody. My husband has raised the oldest girl since she was 3 she is now 13. We both look at her like our daughter. We live in Middle Tn. Is there anything we can do to get custody of the 13 year old that is not biologically my husbands? Her biological father is not in the picture and doesnt want to be. What can we do? Please help! Thankyou