You should be applauded for taking in your 17 year old niece while you are parenting toddlers. But I totally agree to focus on her mental health and well being for now. If she’s still doing well in school and staying on track with that, she is doing better than many 17 year olds. A gap year (or more) is completely fine if she needs it. I would not push AT ALL. Is she receiving counseling/therapy? Many many teens go to college with an undeclared major. Or swear they have a major and change in anyway.
I have a senior with competitive stats that is unlikely to go to a competitive college for reasons both financial and in terms of personality. Don’t think that needs to be a thing that happens to be a happy, successful, productive adult. She will find her wheels when she is ready. Teens change so much even from month to month. Focusing on her emotional health now will pay out in dividends later.