According to what I've read on other support websites, etc., it seems to be more common for the golden child to become a narcissist than it is for less-favoured siblings to become narcissists, but it's far from universal. There are many different ways the parent-child relationship can play out here, even in the general categories of "golden child" or "scapegoat" or what-have-you.

The book The Wizard of Oz and Other Narcissists has some decent discussion on the effects of an NPD parent on what it calls "the chosen child" and the "rivaled-against child." The author doesn't name either as more likely to develop narcissistic traits or full-blown NPD, but she does believe both tend to be more damaged than children who were generally overlooked by the parent and more on the sidelines in the family. Worth a read if you're interested.