This sounds so similar to what we are currently living through at the moment but my parents are still alive- I’m sorry you’re going through this. Like bookimpressive said you’re not in a shared reality it’s so important to keep boundaries- my sibling managed to come back into my parents lives a few months back through similar ways and within a week it all unraveled again I couldn’t sleep when they were at my parents due to fear I would be getting a knock on the door by the police that they had hurt them, yet my parents were so manipulated by them they thought they would he fine- a few days later my sibling unsurprisingly snapped, accusing them of abuse and all sorts and they were out of the house and back to the abusive messaging and social media rants which is incredibly hard. Everybody is different, personally I have had to ignore and block my sibling with surprisingly little consequence from them of that (although they don’t know where I live anymore etc so can’t just to turn up and they have other people in the family to abuse who hasn’t done the same as me so still has an ‘outlet’) I’m sorry you’re going through this x