My sister and are are very similar, but also polar opposites in world view. But there is so much more to that story than just having different world views. I made the decision to disconnect after suggesting we try family therapy, and being rejected, ultimately it was very good for my mental health and I feel significantly better overall.
I had been in a similar place with my mother, however after a year of family therapy with her, we have worked out our issues and become better people for it. So I don't just write people off, I certainly have talked about my decisions with my own therapist, and those were the right decisions to make at the time.
I'm sorry to hear that was your experience. With the right therapist that works for both parties involved usually you can make progress. For me I never made an ultimatum, but I knew from experience that therapy would provide us with a safe place to talk and listen and hopefully find common ground. Unfortunately with my sister ,talking one on one had not worked, so I felt like I only had one last option in the bag.