User blog comment:B214/Question & Answers 5/@comment-26510794-20150816054944/@comment-26510794-20150816055843

well according yo volume 9 i think (i dont remeber actually) they made a test toghuther against the Gremory clan family dressed as the power rangers XD, to see if ise was worth to marry with Rias. And in volume 19 Life 0 Yasaka the mother of kunou addresed Rias as "Sekiryuutei’s legal wife-sama" so i think Rias knows that they are already "married" or enagaged.