However, you never know, and these kinds of assumptions must not stop Christians from doing what they know they need to do and are compelled to do in good conscience. It might be the case that she really does not know that what she is doing is wrong, and your addressing it with her could be the thing that prompts her to act appropriately.
I will share an option with you, which is to talk with her. It might be that you don’t feel comfortable, or not equipped or prepared to be effective. So you might ask her if she would talk to a good priest you know, or someone you know who really knows this subject matter. Or maybe you can give her a good book or pamphlet, or a tape by a good speaker, etc. These are not as effective of a step as talking with her yourself.
Taking the approach of talking with her, the first place to always start with these situations is to acknowledge that we are not God; God is God. So we must not think that it is our responsibility to change the situation, or that it is us who does it. If any good is going to come from this situation, it will be accomplished by God. Maybe God will use you to help your stepdaughter, and maybe He won’t. You are open to it, and that is much.
Secondly, we must acknowledge that the Holy Spirit is working on her and she is where she is in her relationship with the Holy Spirit. It is never for any of us to decide where someone else should be in their journey toward God. We might not like what someone is doing, but only God knows all the facts to judge the person. So you must approach her with gentleness and a posture of respect for her free will and her right to be moved by the Holy Spirit as she is open to it.
Thirdly, we have to have some credibility with the person. If we don’t, nothing we do will be affective. In fact, we could do more harm than good if we are not someone the person respects, admires, loves, etc. So if you have a good rapport and relationship with her, and you believe she would sit down with you to talk, then that is good.
The way we say things is just as important, if not more, as what we are saying. You can’t just sit down with her and blurt out that she is living in sin. That will not go over well. You have to consider who she is and what she might be open to hearing.