Competition between siblings is often used by narcissists to control and dominate the family. Or one child is manipulated into taking the narcissist’s side and may even be used to punish other siblings.
Remember, none of the siblings are at fault. They have literally been made by the parent to act this way.
The ‘chosen one’ often stays on the side of the narcissistic parent, defending them and berating the ‘scapegoat’.
The scapegoat is more likely to recognize that there are problems and more likely to seek help in healing from emotional abuse.
The question often arises about whether the adult children of narcissists who understand what happened should try to educate the others about the true nature of the narcissistic parent. This is probably best considered a personal decision which an individual needs to make for themselves.