Board Thread:Speculation House/@comment-68.186.83.101-20130601225528/@comment-4010415-20150918015814

99.9.180.27 wrote: But they have the same dad, right..? That's what Yang said in the show.. Yes, they have the same dad, but it's possible that he did not marry Summer. Sometimes, when a woman births a child when she's not married (sometimes not even in a relationship with anyone), she gives her child her own last name, instead of the last name of the man who conceived the child.

So, Taiyang Xiao Long (their dad) may have married Yang's mother, thus leading to the couple giving their child the last name Xiao Long. But then her mother suddenly left, maybe even breaking off their marriage. Summer Rose came to Taiyang's side to help him raise Yang, and they ended up conceiving Ruby together.