Update
For those who haven’t read the whole thread, my sister began an online hate campaign against me 2 years ago which continues. I believe it’s because I was FB friends with her current boyfriend’s ex and his ex had been messaging me about his child. But I don’t actually know for sure what had driven her, until now.
Police have spoken to her twice before but she ignored them and escalated to using my real name. She publicly declares that I’m a narcissist/sociopath/psychopath most of that lifted from my DV ex’s hate campaign which occurred at the same time as her’s.
Anyway, despite my other sister and my DP explaining that she is probably doing all this out of envy I ignored them and tried to find a reason for her behaviour, but it seems they were right, as the policeman also explained.
My sister also did this to her boyfriend’s ex, and is quite a jealous/vindictive type character anyway since childhood. Most people know this about her, and she’s enabled my our Mum who seems to enjoy her middle child’s bad behaviour. But my Mum has a history of abuse herself and didn’t treat me well as a child, so my DP and the policeman suggested it’s because Mum feels guilty and is in denial or something. Mum also allowed my violent DV ex to lodge in her house recently. Invariably he then found out where I lived and came round makin a scene in front of the children until the police arrived.
Anyway to summarise, policeman said anyone is free to post anything derogatory they like in their Facebook profile, as long as it isn’t targeted at a specific person in a public forum.
Although my sister has done that to me in a public forum too, I’m not going to have her arrested for this reason:
After a good hour or more, he explained that to any outsider it is clear she is envious of something she can’t have.
My sister has always had good careers, nice home, high maintenance with her looks, once married.
But she now lives on a rough council estate, is being evicted,has completely let herself go in terms of looks and maintenance, her boyfriend is a weird Walter Mitty type who pretends he invented random things like stair gates and talks about roadie-ing with famous bands (forgetting he would have been about 5...), she is long term unemployed, lives off Mum financially, etc etc
She was also rejected by my current DP in the past and just before he met me.
Although I wondered if she was driven by envy, as I’m a poor judge of character, it has finally taken 3 other people all saying the same thing about her that has convinced me.
She was never envious in my lone parent years with a toddler and baby, when I’d fled DV and was barely affording to work juggling childcare costs though 
although she was still spiteful then, shutting her door in my face when I collected my kids if they’d been visiting, taking my bank card to a cashpoint to borrow a tenner but taking thirty, ..
Anyway, I realise now she is doing these things because her life is at rock bottom, and hopefully when she gets up and running again it will stop. Having her arrested was probably pointless as she’s proved retaliatory vindictive behaviour before when I’ve said I’d have to involve police if she didn’t stop. Plus the fact that is know realise she must be depressed as hell otherwise why behave so badly.
I’d already read the stately homes threads and domestic violence threads over the years after my experience with the father of my children, and I should have recognised her character traits in there, but I think like with domestic violence relationships, it takes several attempts, or several years, before you finally reach an epiphany moment, and realise they are the weaker person, not you.
She’s still carrying on, posting Mother’s Day photos from our childhood and cropping me out, but now I just feel nothing, or rather, I feel sorry for her, not in a patronising way, but I really hope her life improves.
Anyway, so the law states people can write whatever they want under certain criteria, and you have to accept it, if it’s someome’s opinion and doesn’t cross certain boundaries. My dv ex hate campaign crossed boundaries so he was dealt with officially, but my sister keeps within them so she continues her campaign.
It’s the end for me of a very stressful few years.