A 17 year old may be considered an adult and many courts would consider her an adult if the parents know where she is and her whereabouts. Does she work, is she out of school is she still attending school regularly? Many factors determine whether she would be considered a "runaway." I would encourage her to stay home and try and help her in anyway you can.
She's a child for family law purposes until she turns 18 or is otherwise emancipated, e.g. gets married, joins the military, etc. Your parents could consent for her to move out, but if they don't, odds are they can call the police, report her as a runaway, and have her brought home. Also, whoever she moves in with can be charged with harboring a runaway.
I agree with Mr. Payne. Encourage her to cool her jets and stay home until she's 18.
Your sister must follow the dictates of your parents rules about where to live. If your parents give her permission to live elsewhere, she can. If they do not allow it, then she cannot. If she leaves the home and stays away without permission, your parents can report her as a runaway and anyone who provides her a place can be charged with harboring a runaway.