User blog comment:Endor 66/Why do Ruby and Yang have different last names/@comment-15108745-20140225225044

There's been a few theories though. One was that Ruby and Yang are half sisters (which hasn't been denied by Monty yet) and that Ruby and Yang might share the same father but have a different mother or vice versa.

The other is that Ruby and Yang were born to the same parents and then the parents split up and each child grew up separately. Hence the different last names.