Obtaining POA for 89 yr old Mom and dealing with difficult sister. How do I prepare myself for what looks to be a battle royal?

The on-going saga. I live with my 89 year old mother. I'm 65, retired, divorced no kids, good physical health and love my mother. Until recently, it was not a problem as mom was quite functional. She could walk without a cane or walker and drove short distances. I moved in with my mother after my divorce 20 years ago.
In the last month and a half, everything has gone downhill. My mom suffered a compression fracture in her back and became bedridden with extreme pain. On top of that she had a bad reaction to an insomnia drug that caused her to have an acute memory loss. I took her off the medication ASAP, but since then her memory has deteriorated. She has lost some of her balance and I feel she cannot be left alone. She gets through the house by furniture surfing which is possible since its a small house. She has had several falls.
Because of weakening condition, some cognitive loss and general uncertainty, I had her MD give a test for acuity which she passed., with some errors. I have talked to an attorney regarding a Power of Attorney (POA), in view of her deteriorating condition.
My main problem now is with my sister. She is 63 and lives 10 minutes away with her husband and one adult child. She is a bitter, unhappy person, whose main hobby when talking to my mother, is recriminating her for all the "things" my mother has done to her. She is either scolding her, railing against me (we have a rocky relationship), or berating my mother for some imagined fault. When my mother hangs up the phone, she is always upset. Her latest topic is the upcoming marriage of my niece. This is all she is obsessed with at the moment.. She refuses to come over and look in on mom. She refuses to recognize that mom is 89 and in poor health. She can only talk about herself and her children. She hates all our relatives for the imagined faults they have done to her. My aunt calls her a witch. I feel she has mental issues, but she refuses to seek help. She is the ultimate "Victim".

My biggest fear is that if she finds out I have a POA on Mom, she will go ballistic. I can't get her to sit down and talk about Mom' s problems, without degenerating into a screaming argument. How do I prepare myself for what looks to be a battle royal? I don't want to argue or expose myself to more abuse from her.