I'm filled with dread

Hi. I just don't know what to do, and this empty white square feels like desperation without hope. In short, my sister has some kind of psychosis and it has been large or small in our lives for years. It's been getting worse, we didn't know it was a full on psychiatric issue when she started. When small, it exhibits as paranoia, certainty that everyone is slighting her, and vindictive get-backs. When large, it's appalling confabulations about family members (imagine the worst, none of it real), abscondment (for weeks, careering round the country), and I just can't tell you the all of it. Anyway right now I haven't seen her since pre pandemic (I'm here, she is in the UK where I come from). I call her, she never calls me. She's always "about to go out", "someone at the door", and the usual avoidance. I can't speak to her straight like I would anyone else, because I tried it, and it doesn't reach. She refuses messaging apps or other online (in fact she's on Facebook but specifically for me she plays the 'like everyone else's comments but mine' game. My cousins notice this, it's not me being weird! So, I several months ago I made plans to visit her in a few weeks, and see some other relatives. Usually she has been looking forward to seeing me but as her illness has progressed, I have joined the ranks of the excluded. I suppose I should be pleased I hung on as long as I did. Two days ago she found an excuse why she could not be available on the dates we agreed (it's only four days), so I changed everything with the other relatives, to see her on a different few days she wanted instead. Now she is telling me I have to leave after two days, and that she suddenly "has to visit" a research place she goes on one of the other days (clearly time critical!) so I will have to do my own thing. I literally get there one day on long haul, she is out the day after, and wants me to leave the morning of the day after as she "may want to stay over somewhere". She's forgotten she already told me another day she was going to be in some other city, the entire story is just hurtful. This is definitely her illness, she has alienated everyone except me, and now it's finally me. We even had a very terrifying few weeks when she thought her husband was estranged from her (he is not). For this one, there were police reports, ambulance, adult mental health crisis team...we finally got her to her GP, it was a nightmare. Anyway I have coordinated with her husband to keep in touch, but now (I'm sorry, this is about me), I feel absolutely finished. People urge me to keep trying to be there for her. I've done 5 years of one way comms and one way effort and trying to redirect her incidents of non consensual realities. But I have PTSD from burnout (was astonished when diagnosed, I thought it was a war thing but anyway I have a lesser version of it), and her gaslighting is just too much. Gaslight is a word I have to mute online as even seeing it is enough to stress me out to the max. I feel horrible but I dread this visit now, I know she will make even this pathetic 36h miserable for me. I feel like a bad person and guilty that I feel bad for me and diminishing compassion for her. I'm lost. I don't think I ever want to speak to her again (but she's my little sister, I do, just not this version of her). I feel like I am hurting myself every time I call her. I'm not sure I can keep in contact any more to receive this sadness and pain. It takes me too long to recover. Well meaning advice telling me to keep trying is a very high cost. If anyone out there has any insight of any kind, please let me have it.