90 year old father refuses to eat or go to doctor. What do we do?

At his age, he may be closer to dying. When people are in the active phase of dying....sometimes a week to 3 days, they often refuse food.

Still, IMO, if he is aggressive, call 911 and let them take him to an emergency room. They can ensure he is fed properly, and comfortable whether or not he is in the active dying phase.

If you live in the USA and you call 911 from any state. Make sure you tell them that he lives in another state, so they do not use enhanced 911 to send the ambulance to your house, instead of your sisters.

I agree that calling an ambulance might be best in this situation.

Is Dad on hospice? If so, ask the nurse to come. (If not, ask M.D. to order an evaluation.
Can sis call for an ambulance to transport him to the hospital? Are his other functions working? Breathing, urinating, moving his bowels, etc? It could be ANYTHING. I think a trip to the ER might be in order along with a hospice eval after he is discharged.