Can I prohibit my daughters step mom from attending dr appointments?

Short answer: "no."  Long answer: "Maybe . . . ."  Presumably, the step-mom is there with or on behalf of the father, with his consent.  If not, normally doctors will take directions from the parents as to who should and should not be present.  If so, the question becomes why you have a problem with it.  If you believe there is some actual risk of harm to the child (physical, mental, or emotional), youi can ask directly to your ex that the step-mom not attend, and if you are refused ask the court to issue such an order, either on that point directly or giving you complete control over medical matters relating to the child; making medical decisions is one aspect of "legal custody" which a court can divide however appropriate between parents.  For details, see the information, materials, and links posted here.