Hello and welcome to the forum. I'm really sorry to hear about this diagnosis for your sister.
In the United States, doctors are bound by the HIPPA privacy act. They are only to discuss medical conditions with people that a patient has listed on the privacy statement they sign. https://www.hhs.gov/hipaa/for-professionals/privacy/laws-regulations/index.html Was the person that was contacted on your sister's paperwork? If compliance of this law was not followed, you can report it. When you say that the diagnosis hit your family hard, which is certainly understandable, was the doctor contacting someone to make sure your sister was adequately seeking treatment for the cancer (in order to save her life)? Is this person a friend of the doctor, patient of the doctor, in some way affiliated with the doctor? What do you think the intention of the doctor was in contacting someone? HIPPA is important but ultimately, it does sound like the doctor was trying to help your sister. Although, it is always up to the patient about whether to get treated or not when diagnosed with cancer. Not defending the doctor's actions in anyway. Private information needs to remain private, hence the laws surrounding this. Just trying to understand the dynamic to best answer your question.
Your sister got second and third opinions confirming the cancer and is undergoing treatment. Surgery and chemotherapy are the typical route for treatment. Ovarian cancer tends to be a 'seeding' cancer in that tumors can be microscopic and surgery alone will not catch them all. So, typically more than surgery is recommended. https://www.mayoclinic.org/diseases-conditions/ovarian-cancer/diagnosis-treatment/drc-20375946
Again, so very sorry for this diagnosis of your sister. That's hard and we are here to talk to you as your family goes through this. All the best to all of you.