Board Thread:Speculation House/@comment-174.88.131.94-20131012165729/@comment-71.227.243.152-20140817152919

Well yang and ruby are half sisters so they must share the same father (if they are actually related) and seeing how Qrow is their uncle, Qrow is the brother of their father... that out of the way, said father may not have married either yang or ruby's mothers and thus they have different last names. on top of that he may have onl married one of them and we would find out that by learning Qrow's last name.

tl;dr: Ruby and yang probably have the same father and we will find this out if we ever learn what Qrow's last name is. (or if monty desidse he wants to shake things up and change everything on us.)