I'd actually agree, toffee.
I'm afraid that in the case of the first relative, now deceased, I was not willing to be attacked physically and verbally over and over. Think hands round neck, knife-waving, trying to push people off ladders.
The other one is much less dramatic, but she turned against me over an entirely imaginary slight. Provably imaginary, fortunately; it does help to have solid proof when it comes to the "he says, she says" thing. After 6 months of quite appalling emails from her, I said that I didn't want to hear from her again until she was willing to discuss things in an adult manner. Still waiting!
In the case of the third relative, I'd love to have a closer relationship. Unfortunately like everyone else in his first family, and almost all his friends, he's allowed them to be edged out by his wife. He says really quite unpleasant things about people now, I'm fairly sure that it's because he actually feels guilty and is in some way trying to make it -our- fault, because he wants to feel justified. It's pretty unpleasant to be on the receiving end of it though.
For the OP I can really see that she might have come to the end of her patience though, and I said that firm boundaries may be the way to go rather imo.
But I'm afraid that life is too short to have someone in it who is actively making it really rather difficult over the long term by unreasonable expectations. Being on the receiving end of this kind of stuff drags you down much more than you realise.