BIG decision i need help???

I’ve been married with my husband for 4 years now. My husband has a daughter and I’ve know my step daughter since she was 1. My husband was unfaithful to me and we are trying to work things out. I feel confused with working things out and stuff. Now a big issue came into the picture to top it off. My step-daughters grandparents lied to the court and forged my hubby’s signature giving up his rights so that they can get legal guardianship for my step-daughter. My step –daughter biological mother is an unfit mother with drug issues, she’s been in and out of the baby’s life. My step-daughter calls me “mommy” and its something she picked up on her own. We talked to a lawyer and she advised us to do the paperwork to fight back and grandparents will get in big trouble because of what they did but not only that our lawyer advised us to fight back for 100% custody but now im scared. Im FREAKING OUT because I don’t know what is the best decision  I want us to work and I want us to have my step-daughter because she means so much to me but unfortunately I don’t know what to do?  I need suggestions and opinions. Please help?????? (im scared of us granting 100% custody and for us not to work, where will she stand )