If you have the inclination and the wherewithal, help the two younger kids. It sounds to me like your oldest niece needs to experience some hard consequences of her actions before she’ll get any sense. Talking to her isn’t going to do much good. The lessons that sink in are the hard ones.
Good advice.
If you ever want borrow to help finance your child or grandchild’s education, take out an ordinary bank loan. If disaster strikes (e.g. catastrophic illness or disability), you can deal with a private lender. A student loan will follow you to the grave.
I agree with Huntress. Don’t stand between your niece and the consequences. They are bought and paid for, it’d be a shame to ignore them.
I'm in a different situation. My two oldest kids are 27 & 29, one has a degree, one doesn't, but he's going back to school while working full time.
Our youngest is a HS freshman. Thanks to the equity built up in our home, we will have enough money for him to go to college almost anywhere he wants to go. I told him that, with a major caveat: he's going to have to prove he's worthy of it. Otherwise, his first two years will be at a local CC.
Unless something changes soon, he's headed for CC.