Can I claim a child as a dependent if they are not related but are living with me ?

My son's 17 year old girlfriend moved into our home in May 2019. She moved in because her mother and mother's boyfriend are struggling with an ongoing drug addiction. The mother seems to have no relationship with her daughter and never made any attempt to contact me about the living situation. I am taking care of this child as if she was my own without any financial support from her mother or bio father who she has no relationship with. I am under the impression that the father pays child support to the mother even though the child is living with and being supported by me. My question is what are my rights to claim this child as a dependent on my taxes since the child has lived with me full time for 7 out of 12 consecutive months ?