Seventeen is a critical age for fatherless daughters. They either move boldly forward with a solid plan for their lives or flounder while hanging out with friends and boyfriends who also lack direction. If your stepdaughter has low self-esteem like many teens who grew up without dads, she will surround herself with people who also feel bad about themselves. She'll avoid those who are striving and achieving.
Now that she is 17, your influence is not as potent as it once was. Hopefully, you've built a strong bond with her throughout the years, and she has known you as a loving parent figure and a man of honor. With a good paternal role model, she's less lucky to look for love and acceptance from a teenage boy or be eager to have a baby before she's ready.
Unfortunately, some stepdaughters never view their stepdad as an asset in their lives but only as an interloper. They see him as someone who robbed them of their mother's precious time and focus. If there are other siblings, the daughter may feel even less significant as her mom's attention gets sliced up into smaller pieces.
Now would be an opportune time for all of you to attend family counseling together. Instead of making it seem that your stepdaughter is a problem that needs to be fixed, everyone should admit to their shortcomings and make an effort to improve the family dynamic. If the therapist wants to work with your stepdaughter alone, she'll let you know.
It's always useful to get a professional's insight, especially when we can't see the interactions in our own family clearly. It could also help prevent more serious issues in the future.