Is it illegal for a pharmacist to diagnose people as an alternative to going to a real doctor?

My wife's sister works at a pharmacy in the local grocery market. She often diagnoses my wife, and myself, even though she has no experience being a doctor, or any of the credentials it takes to be a full fledged pharmacist. My wife often takes her advice over going to the doctor, despite my objections. Even if she was a full fledged pharmacist, is it illegal for any pharmacist to diagnose people who ask? Where can I find that information for myself?