User blog comment:Zathronas/Analysis by an author: "Gold"/@comment-98.109.162.219-20131227040101/@comment-24218038-20131227162023

Difficult question, I found plenty of evidence that Summer is Ruby's mother but none that says she is Yang's. So I would say no, that leaves them with the same father and different mother.

Actually if you read "Red like Roses pt2" It is only about Ruby and no mention of another daughter.