Braces and other dental procedures are considered medical type procedures, and generally, only a parent or legal guardian can consent.  Some doctors will accept a notarized affidavit by a parent , however, others require proof of legal guardianship-- which a simple affidavit is not.  If your sister is refusing the medical treatment, then she has that right as the biological parent of the child, and that right, trumps your parents... at least for now. If your parents have had the child for three years, then they need to intervene in the custody suit and have it modified to grant them custody on the basis that mom has essentially abandoned the child.  Once your parents have been granted either full or shared custody, then they too will have the legal right to consent to medical treatment, despite mom's interference.