I really think a good start to approaching this situation is finding out how she is feeling about it and get her take without judgment, listen and express your concern in a direct but truly caring way. Meet her for a walk/coffee. Is there mental health/addiction issues? Are your parents seeing some of their help as enabling the situation? If your sister is defensive you may have to back off for a bit, or you might find she is also unhappy with the situation but hasn't found her way out yet...which may be where your support could be helpful.