Can you cite something for this?
https://en.wikipedia.org/wiki/Extended_family
Rather than "everyone is roughly equal" it's probably more accurate to say that the degree of power, dependence and investment between family members varies dramatically from culture to culture. The nuclear model is certainly widespread in modern times, but multi-generational models are still very common, especially in Asia.
The post-familial model, where Thanksgiving becomes Friendsgiving because you don't have any blood relatives you know and like, is surely the newest and rarest. I think having friends who are more important than extended family, let alone immediate family, is a pretty modern phenomenon.