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Yes it is unfortunately common. The family members align in their identification of the scapegoat and their cohesion influences the involved therapists who do not realize the larger picture. The victims are themselves both traumatized and also inculcated family members so it is fairly common that the scapegoats do not realize how fucked up things are until they're older. Incest families are often highly invested in a family ideal that both hides the ugliness and also enables it to flourish (because our golden boy would never do anything like that!). When the scapegoat finally figures it out and tries to tell the truth they are shut down by the defenders of the family ideal. It is easier to shun the truth teller than to accept that the family system is rotten and work to reform it. Also those who are privileged within the family frequently see nothing to gain by admitting wrongdoing and so don't. It is as it is with any sort of privilege at the societal level. Those who have it tend not to see it and then resist giving it up even when the weight of evidence that harm is being done is large.

Is it usual that all the family and even therapists side against alleged victim?

On the family conspiring against a single member part, you might want to look up Harrison Post or watch the "Britney vs Spears" documentary (though that's a bit different and doesn't have to do with rape, Britney was the rich person and her family conspired against her to take over her wealth, had a psychiatrist, James Spar, fabricate a report against her that subsequently just miraculously disappeared from the court files and even picked a lawyer for her who would side with them).

Going back to this story, the "shht, let's not ruin the future of our child prodigy, so even if Sam did something bad to you you will keep your mouth shut" is not that rare among families. Conspiracies by families to cover up rape done by one of them do happen:

• https://www.bbc.com/news/uk-england-nottinghamshire-17474977
• https://www.rte.ie/news/courts/2023/1009/1409829-abuse-sentencing/
• https://jamaica-gleaner.com/article/esponsored/20230418/family-members-force-woman-hide-sexual-abuse
• https://www.news24.com/citypress/news/women-speak-out-on-how-families-cover-up-rape-by-loved-ones-20180808

Another very important point that makes Annie Altman's story highly believable is that since 2020 she's been resorting to sex work (and also begged on Instagram one day) just to survive, while Sam has a net worth of over half a billion. As I said in another comment, if one of your siblings needs to do sex work to survive while you have hundreds of millions of $s at your disposal, then no matter whether the accusations are true or not, you're the bad person and there's something fundamentally evil about you. Going back to the sex work part, here's what the statistics say (translated from German: https://www.rheinmaasklinikum.de/Inhalt/Patienten/_doc/Evangelische/Hintergrundinformationen_zur_psychischen_Situation_von_maennlichen_und_weiblichen_Prostituierten.pdf ):

In Hamburg, 98% of sex-workers examined were diagnosed with at least one traumatic event. 83% had already experienced trauma in childhood (family violence 70%, physical abuse 65%, sexual abuse 48%). Trauma experienced during prostitution was found in 83% (physical attack 61%, rape 61%, threat with a weapon 52%). Here, 53% achieved an illness score in the sense of post-traumatic stress disorder. Abuse and dependence on illegal drugs at 74%. Similar alarming findings can be found in international comparative studies.

And also (https://bmcwomenshealth.biomedcentral.com/articles/10.1186/s12905-017-0491-y ):

Intimate partner violence was experienced by 21.2% of sex workers, and 23.4% reported client physical or sexual violence. The majority of sex workers (71.2%) reported childhood (i.e., when age <18 years) physical or sexual abuse.