I need to know what to do to make sure my step father doesn't have a say in my mother's health

If your mother is capable of signing a document naming you as her Medical Agent, you would have the ability to speak with your mother's doctors to help make decisions for your mother. Without such a document, the doctors look to your step father for help with decisions. Also, if your mother is capable, you should think about asking your mother to sign a Power of Attorney that allows you to help with financial decisions.