User blog comment:Endor 66/Why do Ruby and Yang have different last names/@comment-186.188.68.63-20140226184818/@comment-3146930-20140226204557

Except Ruby mentions in one of the earlier episodes that Yang sounds like their dad when Yang tries to get Ruby to make friends. It is possible that Ruby's mother is different from Yang's, but then why would Ruby's mother hide the fact that they share the same dad; especially if Ruby ended up living with Yang as a child anyway. If Ruby's unknowing dad was raising her as if she were his, what would make him any different from a normal dad, thus eliminating the need for a twist.