As to the "chosen victim" model of family life, it's fairly common. There's a well known pattern involving narcissistic parents, wherein one child is the "Golden Child" and like, the Narcissist, can do no wrong. Another child is chosen as the "Scapegoat." This dynamic happens in families that don't contain a narcissist, but it's very common - diagnostic, really - in narcissism.
There are some clues in this case. I'm not going to quote SM but I will note that the picture trail in this case has bothered me. I'll describe a different case (that is very similar) so that I'm not breaking rules.
In the case of a woman with Narcissistic/Histrionic diagnosis (both a field diagnosis and a clinical diagnosis from a psychiatrist), she had a FB in which you could barely tell that one of her children (the boy) existed. The daughter's birthday parties and other events were displayed on FB almost daily. Tons of selfies with the daughter. Even in travel pictures, one had to wonder where the boy was. The pictures were taken by both parents, but rarely contained any with the boy in them. Boy's birthday pictures were never posted. Since I got to interview this woman, I asked to look at family videos. Huge, extravagant birthday parties for Daughter, some years, no birthday party at all for Son, and when it occurred, no videos, we had to hunt for a couple of photos. While we were doing this, the woman showed no awareness of the issue. She just didn't see it.
The Son did get taken to the doctor and she sought psychiatric treatment for him when he was only 6. She seemed to think there was something wrong with him, although she couldn't articulate it. The father was frequently gone (military) and eventually left altogether, but before that occurred he revealed that the Son was diagnosed with depression at age 6.
The Daughter was rarely punished, the Son was punished frequently, often with psychological pain rather than physical pain.
This case has been published in academic literature, pretty much as I just outlined, so I'm not breaking any professional rules. There were some details about the case (non-criminal) that caused the treating psychiatrist to wish to publish some of his findings. There was, for example, a pretend abduction (of the Mom).
Anyway, there are things about this case that make me think of that case.
