I really am trying hard to keep my stepdaughter from being with her formerly drug addicted biological mother.
Do you doubt that her biological mom will be able to take care of her?
She used to be a drug addict so she can't take care of her right.
It is good that you care about your stepchild. You can take care of her and protect her from those that want to hurt them even if you have to call the police.
Shouldn't I be allowed to try?
Well it makes sense that you are fearful that her biological mom will not be able to provide proper care. If this is your view, maybe contacting the state services would be a good idea if you are required to give the bio mom custody.