Before handing over any personal or insurance information to anyone, I think it would be wise speak with an attorney in your area and explain to them the details of the situation. I'm not a lawyer. That being said, I need clarification on some of the things you stated in your question:
Is your step-niece listed under your insurance? Or did her own insurance company contact yours?
Have you been in contact with your step-niece since this happened?
Have you received any paperwork indicating that your step-niece is attempting to sue you for liability for her injuries?
I could be totally off, but if I'm understanding what you're asking correctly, it seems as though the insurance is investigating her injury because her intoxication may have had something to do with it.