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Original Post
This is really difficult. I haven't had to live with siblings in so long and the last time I did, my older sisters were already out of the house for quite a while and my younger siblings are much younger 10, 11 and 16 years younger, so I was more like their mother.
The best way I've found to deal with family conflict in general is to choose not to participate when things start to get to an unhealthy place. I'll use an example with my mother, because she is actually the least mature one in our family, the one most likely to act as you describe. When we were out to lunch and the topic of my counseling came up, she said defensively, "Oh, talking about your 'dysfunctional' childhood?" It was like a bait for me to start a fight over how things were. I just ignored it and continued to talk about the financial aspects of therapy, like I had been. She tried to start bashing on my oldest sister and get me to join in, but I did the same thing, just made a positive comment and redirected the conversation elsewhere. I do this because I absolutely know she is not the type of person who can be reasoned with. She has her own reality and if you say anything that might contradict it, you're in line for some abuse. When she was abusive as a child (verbally threatening, throwing stuff at people, etc.), I eventually got to the point where I just had to tell her I could not participate in the conversation or situation at the time. It didn't change HER behavior (and if that's what you're looking for, I'm not sure I can help...that's something for her to do with her own therapist). But, it kept me safe from escalations while I had to live there. She might react badly and kick me out or threaten me more, but backing off to a safe distance, so-to-speak, reduced her power to do lasting damage. Now, I'm not saying you shouldn't stand up for yourself, because she is your sister and not a parent with the ability to threaten your survival by removing care from you. But, maybe when it escalates, just say, "Hey, I really do want to resolve this, but I don't want to fight with you, so how about we cool off and talk about it later?" Sometimes that pisses people off, though. It may be that nothing really works and then I'm not sure what I can tell you.
The best way I've found to deal with family conflict in general is to choose not to participate when things start to get to an unhealthy place. I'll use an example with my mother, because she is actually the least mature one in our family, the one most likely to act as you describe. When we were out to lunch and the topic of my counseling came up, she said defensively, "Oh, talking about your 'dysfunctional' childhood?" It was like a bait for me to start a fight over how things were. I just ignored it and continued to talk about the financial aspects of therapy, like I had been. She tried to start bashing on my oldest sister and get me to join in, but I did the same thing, just made a positive comment and redirected the conversation elsewhere. I do this because I absolutely know she is not the type of person who can be reasoned with. She has her own reality and if you say anything that might contradict it, you're in line for some abuse. When she was abusive as a child (verbally threatening, throwing stuff at people, etc.), I eventually got to the point where I just had to tell her I could not participate in the conversation or situation at the time. It didn't change HER behavior (and if that's what you're looking for, I'm not sure I can help...that's something for her to do with her own therapist). But, it kept me safe from escalations while I had to live there. She might react badly and kick me out or threaten me more, but backing off to a safe distance, so-to-speak, reduced her power to do lasting damage. Now, I'm not saying you shouldn't stand up for yourself, because she is your sister and not a parent with the ability to threaten your survival by removing care from you. But, maybe when it escalates, just say, "Hey, I really do want to resolve this, but I don't want to fight with you, so how about we cool off and talk about it later?" Sometimes that pisses people off, though. It may be that nothing really works and then I'm not sure what I can tell you.