Have you tried going to your fathers Drs appointments with your sister? There might be a lot more going on then you know. If he collapses in pain again, I would just call 911 and have him transported to the hospital. You have to be very careful with Narcotics and Sedatives in the elderly. It doesn’t sound like he is of sound mind since she is using her medical power of attorney. Does your Father have dementia? If so you sister probably has read that Narcotics and Sedatives can make it worse, but probably didn’t read not all Narcotics and Sedatives are bad. If he does have dementia, is he the one telling you these things? I know for a fact that people with Dementia say a lot of things that do not happen. My FIL Died of Alzheimers and the stories he could tell, none of the true. He even told of one when I met him at the ER and they put him in a room, started and IV and put in a cather, and when I arrived, he said that no one had seen him since he had been there since morning. He was there a half hour. So there are a lot of factors here that are to unknown, how often do you see him?
These are just some of the things Adult Protective services will ask you. If your father is of sound mind then he should be speaking his mind to his Dr. You can call his dr with your concerns and they should speak with him, but since you do not have medical power of attorney they will not tell you anything unless your sister gives them permission to do so.
Good Luck and you all are in my Prayers