Should I be forced to pay a medical bill for my niece who was a run-away and came to my home and I took her to get treated. I am now being harassed by a collection agency.?

It depends on whether you use "should" as a matter of opinion or a matter of law. It's clear that in your opinion you should not. However, as a matter of law the answer will depend greatly on what you signed and how she was presented for treatment, as well as her age. If you take a minor in for medical care and present yourself as the caretaker and offer your name and insurance information, you may have obligated yourself. If you presented her as the legal ward of another party and gave them none of your info, then you may have grounds to fight them. If your niece is of legal age, she should be the one held responsible for the debt. It sounds like you took the responsiblity for her care as a minor child and therefore may well be liable for that cost of that care. You might consult an attorney and research ways to seek compensation from her parents.