Scapegoat

To add to your theory, I feel like oftentimes the Scapegoat child tends to be the most strong-willed when there are multiple children. If we follow your theory, this would make sense because you would want to break the will of the one most likely to stand up to or oppose you. This both decreases or eliminates the threat of opposition from them, but also serves as an example to the other children of the consequences of going against the Nparent. The other children are also used in triangulation against the Scapegoat to break them, as a multi-pronged attack, making less work for the Nparent and overwhelming the Scapegoat with opposition and mistreatment from all sides.