That’s a tough call to make. I don’t have addicted children, but I’ve seen addiction in adults that started early on in young adulthood…
What stood out for me in your story was how early your niece’s addiction started and how fast it escalated (“using anything she could get her hands on”) and I’m wondering if she /someone in your family has been previously diagnosed with a mood or personality disorder? I heard that adolescence is considered the critical age for the onset of such illnesses, making some young people prone to using drugs or booze to self-medicate…
While I second the idea that people should be held responsible for their choices, (and with limited knowledge of the US jail system) I’m worried if the people in jail can provide the mental healthcare that your niece might need? Without wanting to argue against the painful life experience and great advice that others have already given you, I would think about getting her an appointment with a clinician that specializes in addiction and mental health issues in teens and young adults.