1) If you sister is legally competent, then only she can initiate a lawsuit on her own behalf.
2) If you have guardianship over your sister, you may sue on her behalf. If you believe she is not competent to manage her own affairs, she may wish to consult with an attorney about whether you could obtain power and authority to take care of her. I believe a family law attorney would either be the one to help with this, or could direct you to an appropriate lawyer.
3) If the doctor has violated accepted medical standards in his treatment, he could be sued for medical malpractice for any resulting medical expenses (such as to detox or treat your sister), other out of pocket losses, and possibly also pain and suffering. You should consult with a medical malpractice attorney about the situation to get a sense for the potential recovery. Good luck.