How to Stand up to my sister

I am 27 and my sister is 60. The age discrepancy is because I was adopted by my biological grandparents who are her biological parents. Dad had a stroke at the end of March, and he is recovering those some speech difficulties remain. Winnie had a stroke, we knew that he would regain most of his abilities and enough to be competent. And the week after the stroke, I mentioned to Dad that she had reminded me multiple times about $100 that we were going to pay for a donation for something. Under a lot of stress, I used the wrong verb and said pressured instead of reminded. This mistake caused her to yell at me on the phone for 7 minutes before I finally hung up. Then she came over to the house and started yelling at me and we put Dad on the phone to try to help mediate the situation. She basically blamed me for ruining her relationship with Dad, which he seems to be doing all by herself. I eventually beg for forgiveness even though I knew I was right just to get her off my back

. That night, she also got onto me for things I don’t even remember saying or doing when I was about 6 or 7 years old.

A few weeks after that, and about mid-May, Dad had a bunch of doctor appointments where she would take him to them and he would come home that afternoon agitated. I live with him, and she does not live with us. He would be very upset, but yet he would not react that way around me. I figured out that she was somehow antagonizing him. One really bad incident happened in May and I walked in it to them in the middle of a screaming match and our house we just moved into. I could tell that she was the agitator so I told her to leave and she said well good luck with getting all of his meds and doctor appointments.

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And late May, there was another episode. We were trying to move out of our house, and there was some furniture in that house that is supposed to go to her. The furniture have been sitting there for a few weeks and she had about 4 weeks worth of time to get it out of the house. Dad wanted to get it out so he could try to get a painted to sell it. We told her okay you need to get your furniture out by the next day. And then she started to call me and it’s like what do you mean by this and I told her to take it up with Dad because it was Dad’s decision. She yelled at me and basically said that she been giving us a lot of help over the past few months, which is true, but we needed to help her or else she wouldn’t continue to offer help. When I eventually was able to get off the phone, she started yelling at dad. She blamed it on me because I was the one who said that we need to get the furniture out of the house.

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This past Tuesday, dad had a psychiatrist appointment as part of the process to see if he can keep his driving license. She downloaded the new patient paperwork form from the psychiatrist website and filled it out for Dad. Dad is fully capable of making his own decisions, and she just filled it out for him and I believe misrepresented him on this paper. For example, she put that his temper was getting worse, when Dad has had a string of a few good weeks as part of a stroke recovery and I think the temper issues are largely behind him. When confronted about the form, she started to complain about how it was I would have done the same thing had I had the form. No, I would have let Dad fill out the form because he is fully capable of doing that. She then turned on me and basically said that I had one last chance to save the family and that I’d have lived with myself if I didn’t and she was trying to make me out to be the villain. I am someone by nature who wants to avoid conflict, but I see very little choice and that coming up with her. How would you recommend I stand up to her? My approach has been next time she starts mouthing off, tell her to get out of the house. She is also not happy that I talk about her to Dad, when all I really did was tell Dad that she is approached me multiple times about contacting our lawyer, trying to activate the power of attorney, and possibly trying to control the family’s funds. Dad is fully able to do it himself and her ideas would violate the power of attorney documents.

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Thank you for your advice