The lowest-common-denominator thing is counterbalanced, in non-kinship relations, by the enormous number of potential relationships; you can simply select friends who match your committment. Kinship relations are pre-selected for you, though. Darn.

I think the idea of family primacy is pretty new; I'm not sure exactly how it developed. I know that pre-modern literature commonly depicted the fiercest loyalty and deepest attachments in friendship in a way that you don't see much now. Blood is thicker than water only because we thickened it sometime in the last couple centuries.