Board Thread:General Discussion/@comment-9991778-20130601230059/@comment-10390252-20131119133541

Here's a new theory about Ruby and Yang's relationship. Maybe the reason they have different family names isn't due to them having different parents but because of a cultural tradition about names that's different from the Western/European culture.

On Remnant, it is normal for the firstborn to take the father's surname, irrespective of their gender, and then carry it on to the next generation. This means that Yang has her father's surname (Xiao Long) and her second child will, by tradition, carry that name too (it would be her first child if she were a boy - gender equality isn't quite there yet). As already implied, if the mother is the first-born child of her own family, then her second child will have her surname, again irrespective of gender, so that the family name will endure. If the mother is not a first-born, then the naming conventions are more like those in Western/European culture, although it is still the traditional right of the second-born to adopt their mother's maiden name when they reach legal adulthood, if they wish.

Summer Rose was the first-born child of her family, so she kept her maiden name after marrying. As per tradition, her second child, Ruby, has her mother's surname. Because her mother was the first-born of her generation of Roses, Ruby will likely be expected to pass her surname to her second child too.

This also fits into my theory that Summer and Cinder were twin sisters (Cinder's original name being Autumn). When Autumn cast off her original identity when she turned Dark, she legally changed her surname, as was her right as a second-born, to Fall, her mother's maiden name.

I know it seems a bit of a needlessly complex system but, from what we've seen so far, there definitely seems to be a definite cultural aspect of familial loyalty and pride in Remnant's humans. So a naming tradition that allows for the preservation of family names irrespective of the gender of children strikes me as plausible.