Board Thread:Speculation House/@comment-108.69.217.22-20160216061533/@comment-28288412-20161016200457

If Ruby didn't know that Yang and her were stepsisters, then there is only one reason I can see why:

Yang (and everybody else who knows and got to work with her if she actively convinced them, otherwise no one found it relevant to tell Ruby) is hiding the fact that she is the daughter of Raven and not Summer from Ruby under fear that she will try to help Yang hunt down her mother and put herself in danger.

Raven isn't really related to Ruby, so there is no real reason to tell about Raven being Yang's mother. Qrow likely wouldn't think to mention it and Tai was probably barely in any state to tell Yang after Summer's death (I think that's when he told her) so I doubt he was going to tell Ruby.

But Yang had good reason not to tell Ruby. She probably felt terrible after nearly getting her killed while looking for her mother, I imagine she wouldn't tell about Raven or her search due to the guilt and a desire not to risk her sister's life again. Sure Ruby is becoming a huntress, but Yang doesn't want any harm to come to Ruby because of her own personal quest, if she is harmed while training to become a huntress, that is a consequence of Ruby's desire and journey, not Yang's.