I presume the doctor considers your mother to be incapacitated. You do not have any legal authority to make her medical decisions unless she has given you a Health Care Power of Attorney. If she hasn't, then your father is the legal decision maker. The only legal method to override you father's legal power is to consider guardianship. If you become your mother's guardian, you can see she gets proper medical care.
Guardianship is serious business.
You have to hire an attorney to take the matter before a Judge. At the hearing, you must prove you are not disqualified to act as guardian, and that your mother's rights will be protected by your appointment as guardian. The Judge must find your mother is legally incapacitated. The doctor typically provides evidence of incapacity. The problem you'll encounter is the legal requirement that the Court appoint your father as guardian.