It is difficult to see someone suffering and not getting the help that could stop the suffering. How old is your step daughter?
If she is an adult, there really is not much you can do. Know what the resources are in her area and offer them when she is ready. If you push the issue you will push her away. She needs your love and support. Share your feelings with her in a gentle and honest way. Tell her you love her so much and it is hard to see her unhappy/in pain/whatever fits best. Let her know that she deserves to feel better and there is help available. But say it all in a loving, supportive way not in a way that she feels you are telling her what to do.
If she is a teenager, it is hard to force her into therapy. However, her parent/legal guardian can make an appointment with a therapist and take her to this appointment. Ask to give it 3 or 4 times. Be sure you have found a therapist who has alot of experience with teenagers.
They will probably be able to build rapport with your step daughter and convince her to continue therapy.
This too should be presented with love or it will only make her more defiant. Good luck. Let me know if you have further questions.