As tensions began to rise, a hospital social worker got involved to help us navigate the waters and be sure that my mom was getting proper care. She agreed with my sister that my mom needed to go to a nursing home, which is what happened, and it was a nightmare.
So despite the fact that you and your mom had an agreement and a plan that you would move in and care for her, and assist with appointments for PT and OT, a social worker got involved and put your mom in a nursing home? Maybe I'm off base or things are just different where I live, but I don't think a social worker has the power to do that if your mom has capacity to make her own decisions. Did your mom feel pressure from your sister, and secretly agree to this new arrangement? Were there complications with the surgery, or was it a routine knee replacement?
Even writing this I’m getting panicky remembering how I couldn’t protect my mom, and trying to understand what was wrong with a recuperation plan that her doctor had said was fine leading up to the surgery. I tried to explain to the social worker our family dynamics in private but her eyes just glazed over.
So if your mom's doctor had approved this plan, there is no way I can see that a social worker could over rule a doctor. Was there more to this story? By any chance, did you or your mom try talking to her
doctor about the social worker's interference? I'm thinking that could have ended it... If the social worker's eyes "glazed over" and she did not listen to you, something is off. Keep advocating. Or, is it possible your mom changed her mind and secretly agreed to this new arrangement just to get your sister off her back and make that problem go away?
It just doesn't add up to me that a social worker could over rule a doctor, if the doctor agreed to you and your mom's plan.
Now that your parents are in their 90's, and starting to decline, I can see why you are stressed. You can see the chaos that your sister stirs up, but maybe feel powerless to prevent it?
Have your parent's appointed an enduring POA? Have they already made an updated Will? As for their care, I would have a private conversation with them about their wishes, when the time feels right. Possibly consider recording the conversation for future evidence if needed.
Maybe also accompany them to doctor appointments if they want the support? All you can do, is try to honour their wishes for care, to the best of your ability. If your sister interferes, and your parents allow that interference, it gets more complicated. Then if you have concerns for their safety (physical, emotional, economic) because of her, then you can share that with their doctor, or a case worker, or a social worker (hopefully a different one).
Meanwhile, always look after your own well-being.