If a party has joint custody
that means that they have joint legal custody. LEGAL CHILD CUSTODY
The legal aspect of custody for children and divorce means decision making and the parental authority and rights of each parent. It means having the legal right to make and participate in any material decisions affecting the children. Included are the choice or change of school, college, camp, or comparable summer activity, special tutoring, music, sports, art, dance, and other cultural lessons, psychological or psychiatric treatment or counseling, doctors, and surgeons; notice of illness and injury; access to school and medical records; and all other material decisions affecting the health, education, and welfare of the children. Specifically, legal child custody is the right of the parent to make decisions regarding educational instruction, religious instruction, health care, discipline, and child care providers for their child, but anything relevant to the children could be included in the definition. Legal custody can be granted to one or both parents, but the overwhelming preference is for joint legal custody. The judge expects each parent to keep the other fully informed and to consult with the other parent in all major decisions effecting the child.
The medical provider is required to provide a parent with joint legal custody full access to the child's file. The step parent would not have that right even with a "release".
Normally you would provide a copy of the order to the medical provider.