Firstly, it is frowned upon to treat family.
Doctors are bound by confidentiality not to reveal medical details about their patients but they are also bound by a responsibility to society to protect it from danger. If the doctor is aware of a diagnosis that the patient has kept from people who might need to know (eg epilepsy being kept from the DVLA/CAA) then his duty to society probably outweighs his duty of confidentiality to the patient.
If you want a specific legal opinion then ask a lawyer though!!