I think your only responsibility is to make sure she does everything to preserve opportunities. Take the SAT and ACT. Make sure she completes all requirements to get into a state school or to get a state scholarship like the A-G requirements in California or qualifying for Bright Futures or the HOPE in Georgia or Tennessee. Keep good records for things like community service hours, the names and contact info for teachers who could write letters of recommendation if she decides not to go to college right out of high school. She may already have attended 2 or more high schools, and having a notebook of things she did while in high school makes filling out those applications easier. If she takes a gap year (or two) having all the info really helps. Memories fade fast.
If your guardianship is an informal one (if the state didn’t take custody of your niece), you might want to look into making it a formal one through the state. When it comes time to fill out financial aid forms, she might be in a better position to get financial aid if she is ‘independent’ and not dependent. If she’s still a dependent, she’d have to use one of her parents’ financial information and of course find that parent. That may be hard. Having the state take legal custody often has to be done before the child turns 18.