Namechange for this. Background: am one of four siblings (two DSis one DB) all in our 20s/30s. Parents together apparently happily for many many years. All got on well.
Then over about 5 years Dad really changed - paranoid, angry, fixated on money, would spend hours on hours ranting and raving at Mum/us about how we'd all ruined his life. Very unlike him. Very upsetting (obviously). Massively confusing and upsetting for all. Plus memory issues, and seemed unable to follow a conversation any more. Suspected some kind of... I don't know, mental illness? Early dementia? But there is not much you can do with someone who won't listen to concerns, won't see GP. Believe me I tried. Mum actively scared of him due to yelling/aggressive behaviour - he was always such a gentle man 
Anyway after years of this Mum moved out, they are now getting divorced. Dad seems to have got a bit better (can sort of have a conversation now, memory issues improved) but now just pours all time/effort into getting as much as he can out of the divorce. It has been dragging on for years now, and he is still such a ranting, angry bully about it. (Eg - thinks me & bro/sisters should pay to financially support (former SAHM) Mum so he can pay less in settlement, because he "spent so much on birthday presents for you four when you were little, it's the least you could do").
Anyway so it's all a massive fucking mess. And I appreciate he sounds horrendous from this but he genuinely wasn't for most of my life - he's like a total stranger now. It is awful and confusing and bloody exhausting to deal with.
Anyway: current issue is that youngest sis is trying to build bridges with him again and wants me/bro/middle sis to join in. "What if he dies still estranged from us?" So she wants us (especially me, I was youngest child and arguably the favourite) to phone him every other day to try to "cheer him up", update him on his two DGC that I have (who he shows close to zero interest in and wouldn't recognise if they passed him in the street), and generally chat in a friendly way like nothing ever happened.
Other sis and bro are sort of on board with this in theory. Sis #2 has health issues atm and is opting out on those grounds. Bro thinks I don't need to be all chatty with him, but could at least try to reason him out from his little fortress of paranoid ranting. (Which means hours of arguing with him as he gets angry and yells at me.)
But I just don't want to. I feel so so very sad about all that's happened, I'd hate it if he stayed estranged for the rest of his life, especially if he's ill, but I have just HAD it with years upon years of dealing with this. Trying to reason with him, trying to get him to get help, trying to be a friendly presence in his life, trying to get him interested in my DC, trying trying trying. I've just got nothing left to give any more.
AI being totally U and a bitch for all this? Has anyone else dealt with similar? I feel like I should never give up on my own father, but I just can't take any more of this 