As punishment for not making himself a martyr by refusing blood and likely dying, it seems your sister now wants to ensure your father will die a spiritual death.
With regard to his right to privacy in his care, did your father or mother disclose to anyone that he accepted a blood transfusion?
While the choice of medical treatment is a private matter between a patient and their health care provider(s), if your father provided private information regarding his care to anyone not entitled, then he can not reasonably expect to claim a privacy breach for his treatment details being known by anyone he told.
Confidentiality of patient information is a requirement for health care workers, not friends and family. While your father and you may reasonably expect your sister from love will respect his right to privacy, and respect his right to do as he considers appropriate and as his conscience allows, there is no HIPPA accountability that applies to her as family.
As any JW is fully aware, WT requires JWs to report other members to the elders if they are observed doing anything considered 'wrong'.
Since he made a private decision to accept a blood transfusion(s), it would then have been in his best interest to say nothing to anyone especially family members and JW visitors, and to also require any transfusions to be administered in private with no visitors present.
If your father is later questioned by JW elders, he could state his treatment is a private matter between himself, his health care team and Jehovah. If he chooses to admit anything, he could say that he accepted blood fractions and not define what those fractions consist of.