Here are some reasons to think that Youzi did not regard family as the root of humanity or of the Way. (I used to think he did.)
Most of my argument focuses on defending a view held by Soothill, Leys, Chin, and maybe Lau and Slingerland: that by 弟 in Analects 1.2, Youzi meant elder-respect, a virtue commonly associated specifically with life outside the family. It would follow that according to 1.2, only one of the two parts of the root of humanity is specifically a family virtue. If 孝 and 弟 have something relevantly in common for Youzi, family isn’t it.
Youzi says, “One who is filial and respectful of elders, but likes to go against his superior, is rare. One who does not like to go against his superior, but likes to stir up trouble, has never been. The junzi works on the root. When the root is established, the Way grows. Filiality and elder-respect are the root of his practice of ren, yes?”
My main argument for this reading of 弟 is simple: it seems unlikely that Youzi would offer as the root of the junzi’s humanity a combination of virtues that is obviously unavailable to eldest sons. We should not read Youzi such a way as to imply that eldest sons cannot have the root of humanity, if there is any other way to read him.
I’m not sure why this argument hasn’t impressed itself on me earlier. Is it new?
I’ll supplement it in prepared comments below.