She can because she did. Typically, the hospital is more concerned with providing medical treatment than trying to determine whether the person who brought the injured person in has legal authority to complete forms, etc. The step-mother did not violate any laws unless she is a named party in a custody case involving your son and has been court ordered to include you on all forms. HIPPA rights usually protect records from being released to people and are not really designed to assure that all those who have authority to obtain records are listed. This is more of an issue you should be consulting your ex about so that he can discuss it with his wife and request that in any future similar situations you are also named on the appropriate charts and forms.
Sign up to receive a 3-part series of useful information and advice about child custody law.