It is bad enough that the scapegoat in the family has to suffer such abuse. In addition to that, no one in the family acknowledges that there is anything wrong with the way the scapegoat is treated.
The scapegoat can be abused by the narcissist right in front of other family members and no one seems to see it. They all share the reality created by the narcissistic parent.
In cases of a narcissistic ex husband, the mother of the children can be put into the scapegoat role by the father. He gets fills the children with his lies about the mother and causes them to join in his shared psychosis that their mother is the abusive spouse, rather than him.
This can also occur in reverse, where the narcissistic wife makes the father of her children the scapegoat. She lies about him and makes them see her treatment of him to be self defense.
Much of the abuse occurs when others are not watching. When an abusive conversation is restarted by the narcissist, in front of people, the scapegoat naturally reacts to try to defend themselves. But without the entire first part of the abusive interaction, the scapegoat appears to be overreacting and mentally unbalanced.
The narcissist just plays the victim and claims to be doing their best to deal with the disturbed, seemingly inappropriate behavior of their victim.