I agree, she may be on the adventure of her life.
And it's called the School of Hard Knocks.

I'll bet at the center of her disobedience is an individual who feels being responsible has no righteous benefits. All she has to do is look around and see everyone getting away with almost everything. She may feel like she has no future (and she may be right.)

To a young mind with no sense of self-control, I can see how this would encourage her.
She NEEDS to know, say perhaps ....her parents disapprove of her current lifestyle but that they should leave the door "open" for her at any time in the future. Meaning if she gets hungry, bored, sick or has had enough of the deadbeats she's hanging around with, that she can always return home, no questions asked. And stat over. Without the lectures.

It takes a special kind of parent to be patient, helpful and non-judgemental. Your niece may not even WANT to be anywhere near her parents in the first place, but someone should put the "safe" bug in her ear.
Why parents spent 15-18 years raising kids only to let them slip through their fingers during their teenage years is beyond me. I have seen it happen though, to some of my friends or neighbours, and the parents weren't just willing to buckle down early enough and get in their early and REALLY help them. But it's easy to do...to be disconnected from your kids when your own problems are tough. Gotta larn to juggle and juggle hard. And then some.
TOUGH LOVE.