As the son of one who's in fact gotten even more abusive this year, as my father got early stage Alzheimer's, which to date is still a minor disability, to more comprehensively take out 57 years worth of grudges against him, to the point of the unforgivable, to the point I'm almost certain she wants him dead if she can arrange it through sins of omission....

Yes, there are people who you're technically that close to, who you are obliged to "honor", to whom you (eventually) owe nothing, and like in this case, who fully deserve to die alone, and most decidedly unloved.

Although I'll draw one distinction here, it's the sins against another that crossed the final line for me, the one where I've sworn to never speak to her again (modulo someone else's life being in danger). Sin against me, that can be not just understood but forgiven, but against an innocent (enough) 3rd party is somehow different.

(And for a bit more pathos, by her order in 20 days I'll be completely out of the loop and living in the house built in 1910 I recently bought and have been remodeling, seeing as I'm the only sibling who's in the same town, have been living with him for the last 5 years on land she alone owns, and is the one most strongly objecting to her treatment of him.)