I have had guardianship of my step daughter and she has lived with me for the past 3 years without any financial support from anyone, can I claim her as my dependent?

I ended up being left with my ex's kid when he left 3 years ago. I have never gotten any help or aide and I am the only source of support for her. I just found out that her Mom who has not seen her in over a decade has been claiming her because I did not but I just heard I can. Is there anything i can do to claim her as she is my dependent.  I can prove it with school records etc.