User blog comment:Ajrep1994/Yang and Ruby are half sisters/@comment-11188061-20140724002358/@comment-3146930-20140724004318

From what I remember over he course of actually paying attention to what people said Monty said:
 * First mention was that what Ruby said in the Yellow Trailer had meaning (Ruby calls her "Sis.")
 * Then I remember Monty saying that they weren't adopted, they are blood related, but aren't cousins.
 * Then in the show they talk as if they have the same dad, but a mom is never mentioned.
 * Then whenever the topic of Ruby's mother came up at RTX (when talking about the songs), Yang is never mentioned.
 * Yang is never mentioned around Summer Rose in general I think.
 * Then we have today's confirmation

However this is just what I remember the timeline being and I am most likely wrong or am missing something.