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.