You are 100% correct to be concerned for your stepdaughter. I agree that an emergency appointment needs to be made. It's disturbing that this is actually patterned behavior now and she's only 12 years old... but what's more disturbing is that your husband isn't worried about it. It's hard to criticize him for his denial of his baby girl, and he's probably creating logical explanations in his head for her actions.
Do you get any say at all in your stepdaughter's care and appointments? Or do you HAVE to go through your husband and get his approval first? Do you both have to go through your stepdaughter's bio-mom?
I do wonder if there's more at play here than ODD and ADHD. My eldest daughter has those, and is 12, but we don't deal with anything else besides typical tween girl middle school issues. Typical girl drama. If her mother used drugs or alcohol during her pregnancy, this sort of thing may have added some deeper elements such as fetal alcohol syndrome and therefore some serious neurological issues.
I think it's very important that she receive a new neuro-psych evaluation from a psychiatrist that specializes in neurological and behavioral and mood disorders.