You have my sympathies, OP. This sounds so very much like my mum, and if not for a few details being different, I would think it was.
My mum married a man who abused one, possibly both of his daughters. One hasn’t spoken to him since shortly after he was arrested and the other only very occasionally every few years. They were late teens at this time. My mum has said awful things about them over the years and has created a whole narrative (that he hasn’t denied, because it’s useful and distracting) about why they are NC to due with greed and money and their mum not getting as much as she wanted in the divorce, etc. No mention to anyone of how he sexually abused his own child. She does keep an eye on them on social media and tries to get details about his grandchildren (who he’s never met). It’s a weird and unhealthy obsession. She hates them (she’s only ever met the one daughter once for about 10 minutes), but also posts all sorts of rubbish on social media very performatively about how blessed they are for ‘their’ daughters and grandchildren - a daughter he molested and the grandchildren neither of them have ever met. 🙄
Interestingly, since I found all this out and went NC with them both, the same thing has started with me. Lots of social media stalking, harassing messages and ‘stories’ about my family and I that aren’t true. They’re almost exactly the same lies spread about his daughters. Right down to the ‘reasons’ we are NC, supposedly.
My advice and what I’ve done is to lock down all information about yourself - social media, public details, electoral roll, etc. And deploy your own flying monkeys.
I have friends and family who do still engage with them to a very small degree. I’ve made sure they know the truth and I ask them to correct any information out there about me that they hear that’s false. I also live my life quite honestly with people and they know me well enough to know there’s no way I would have done the things I’ve been accused of doing. Actually when I started talking, it turns out lots of people who had been hearing all these stories about his daughters all these years kinda thought they were a load of rubbish. They listened and nodded along, but they had a lot of doubts and questions. It turns out that people sniffed out the situation pretty accurately and could see right through the misinformation campaign.
I get it though because it does shake your sense of self. Even when you know something is untrue, you question yourself and your behaviour because it all sounds so outlandish that someone could be as crazy as they seem. Maybe I’m the crazy one?! It’s only when I occasionally tell the story to someone else that I realise how batshit they actually are and that it’s most certainly not my fault. It’s horrible to think that someone out there, even someone who doesn’t know you, thinks you’re a terrible person. I’ve had friends of my mum who I’ve never even met get in touch to say how awful I am and how I need to apologise for everything I’ve done to them. It’s really unsettling.