Yes, I am 99% certain that she is a psychopath. I've knew her from a baby and her mother had her prematurely which, I think, coloured her attitude towards her. The eldest child got a lot of special extra attention that was very obviously not directed towards her. However, whether it started as a reactionary thing or whether her mother realised what she was very early on and subliminally rejected her, I can't say. Her mother clearly felt a lot of guilt for how this child acted towards other children and didn't discipline her when she acted (very) badly.
What I can say is that she was extremely hard to deal with from the start: p*ssed over every surface long after she was potty trained (including on my daughter), would turn every single drawer and cupboard's contents out and trample it onto the floor, break things deliberately or play with them so savagely that a breakage was certain and she would steal things constantly. The stealing was noticed when she was a very small child, at nursery care, but that seems to be a family trait. As time went on, we would dread her coming around because both she and her older sister would make our children (and any others nearby) argue and cry and storm off at the least opportunity. I was trying to manage the situation with her mother who was also a very difficult character and prone to flying off the handle in cold rages but whom I also felt sorry for because she clearly had difficulties she was dealing with. And her eldest claimed my eldest was her best friend so they 'had to' associate.
Then they moved into our village without announcing it. They'd been turning up on Saturday mornings unannounced for some time, going to a local shop, and would then spend hours with us. I'd be downstairs making small-talk, listening to the screams, bangs and cries from upstairs and wondering when to intervene. I left it far too long but, when they moved in to the village, it felt like our fate was sealed. The youngest child then started a campaign of absolute menace. She would relentlessly bully my youngest, whisper genuinely appalling things to her, hit her, steal her things, line up other belongings that she was to donate to her when she went home (because caring is sharing) and choose precious things to break (most of this I learnt later). When my child finally reacted, her mother went apeshit on her and had to be restrained by another parent.
I finally called things off with them all shortly after this and forbade them from coming around all the time, definitely not going upstairs if they ever did 'drop in' and not responding to invites out. I know her mother to be a narcissist because so much of what the kids said came directly from her mouth and their entitlement to other people's belongings, time and lives was unbelievable. The bullying to my children started in earnest and the mother teamed up with another awful local family to provoke me into responding. Then she turned on the waterworks, pretended it was me causing the problem and called the police to warn me off.
I've heard since how other people have banned this child from their houses or friendships with their children. Their pet cats have gone missing (and I wish them luck - if they really did escape - because the way they treated their poor dog was appalling). I've no doubt we're the villains in their story but anyone who meets this child has exactly the same reaction. It's a visceral dislike that you're almost guilty for feeling, but not quite. The glee I spotted a couple of times in her face when she had provoked another child (often my own) into tears of rage and humiliation was unnerving. I've no doubt what she is and I suspect the rest of her family knows too, but they're deep in denial for all sorts of things and their mother is an absolute horror to deal with (also deeply abusive to her partner), so she won't address it and neither will they. I fully expect this child to develop into something truly unmentionable and, if it wasn't going to be a wider problem than just theirs, I'd think they deserved it.