Sister refuses to get mother treatment. What can we do?

Is the Aunt POA? Or are you? If you are POA you are in charge. Does your mother live with this aunt? I don't understand, if you are POA, where the aunt's opinion figures in anything at all. What am I missing?
I agree with Barb's advice. Call 911 for transit to the hospital and call APS about the Aunt if she is POA.

What Barb said.

When that's done and your grandmother is receiving medical treatment as per her fundamental human rights, after all that, about your aunt.

[pause while I sweep rude thoughts out of my head. Probably been implanted there by all those weekly PCR tests I've undergone in the last few months]

I don't know how you get hold of them in the States, but there are PCR tests you can do for yourself, at home; and perhaps if you get a few kits you can show your aunt the overwhelming absence of anything sinister in them. She can examine the cotton swab herself, dissect it, scan it for secret devices if she likes. I'm afraid there is no convincing some people who will believe only what they want to believe, but you never know.

[brain probe my foot. Sigh.]

Advise your aunt that if your grandmother should pass away because of side effects of the stroke, she will be liable for her death. If your aunt is not of sound mind (and it may well be that she is mentally ill if she believes COVID testing is a brain probe), then your grandmother should not be taken care of by her. Do you live in the same city as your grandmother? If so call 911 and have your grandmother taken to the hospital.

Call 911 and have grandma taken to the ER.

Is she grandma's POA for healthcare? If so, call Adult Protective Services.