I am the co-guardian of my grandmother. I, alone, also hold medical POA. If there is a dispute as to a course of medical treatment, who decides?

As described I was initial given both medical and financial POA from my grandmother. Some time after that my sister and I was given co-guardianship of my grandmother. My sister is adamant that my grandmother NOT be given the COVID vaccine. My sister's rationale against it comes from religious radio shows and other unknown sources. I'm currently undecided but am leaning towards giving her the vaccine, once more is known. Especially if the facility she is residing requires it. Who has precedence in the medical decision?