User blog comment:Ajrep1994/Yang and Ruby are half sisters/@comment-3257939-20140723235141/@comment-11188061-20140724034216

Actually, Yang will be the surname. Really, it can be something childish like, 'Hey, I can't be called Ruby Yang since you're already Yang! So I am Ruby Rose!'. It might even be convenient for Qrow and the adults, since there seems to be some great secrets in the past.