Others have said it all as to why, sometimes, the best and only real option is to cut off contact. For me, this decision came over 12 months ago in relation to my parents, and then, much more unexpectedly, at Christmas time in relation to my brother. I still have sporadic contact with my sister, but quite frankly this relationship is almost as toxic, just slightly less dangerous for me, which is the only reason that communication channels are still officially open.
Yes, I agree with MTS, somehow, logic aside, severing this most fundamental link has probably been the hardest decision of my life, both in terms of following through with it and then living with the consequences. The depths of that consequence are deeper than I could have imagined, yet I tell myself every day that my life is better, freer, safer, more forgiving and kind to me, without their presence.
Creating a non-biological family is something I have been privilleged enough to have been able to do in some sense, though I am yet to experience this reality on an emotional level, rather than merely an intellectual one. There is no easy way through this process. I can only offer thoughts and strength to those who are forced to take this route out of sheer self preservation and an awareness of their right to live a better, safer, freer life.
Maddog