She’s only a junior. My kids didn’t even think about college until spring of junior year. And they ended up at top colleges. It’s fine to wait until summer before junior year or even later if that is what suits her emotional timetable. It is also fine to do community college and then transfer, as many are doing these days for financial reasons.
Given the situation, I think you might need to change your priorities. Settling in with a new family for a 17 year old takes huge psychic energy. I think your expectations are a bit high right now. There is just a little judgment coming through in your post.
Get a copy of “The Myth of Laziness.” (maybe also be on the lookout for any need for evaluation.) I found it helpful.
Or maybe go talk to a counselor to help YOU through this transition. It must be very hard for you and your family and it is apparent you take the responsibility very seriously.
Is she seeing someone?
Just know for now that nothing needs to be done urgently and you can relax and learn to live together.
Whatever the reason, you are a wonderful person to take a niece in.