Can I get custody of my step daughter that I have raised for 16 years?

Question Details:

I have raised my step daughter and now her mom are getting a divorce. She is trying to take her daughter from me all because my girlfriend and I are getting serious and she doesn't like it. I never legally adopted her; her mom never let me. We have 3 other children together. I just want to know can I get custody of my 16 year old step daughter or does her mom have full control of this situation. Is there anything I can do? Important Notice