Hello NikNox, welcome to SR.
I don't know about the law where you live but perhaps you can find another social service agency in addition to NACA; where your stepdaughter (plus you & her dad) can get some counseling and support.
This way there would be a third party to witness what has been going on. Perhaps the NACA can direct you to more resources? Contacting an attorney might also be a good next step.
In additon to coming here to SR you might want to try some Al-Anon meetings; it's a great means of support and information. Your stepdaughter could benefit from some meetings as well; Ala-teen would be a good place to start.
Your stepdaughter is fortunate to have two caring adults such you and your husband, in her life.