User blog comment:Wingzard/Did U Guys Join the Q&A on Twitter?/@comment-7838080-20130618221310/@comment-10488475-20130619013950

They're sisters, but not blood related and don't share the same last names. So one of their families adopted the other. I'm guessing it was Yang's family that adopted Ruby and Ruby's parentage will be hinted at early on and finally revealed later in the series.