Curious that she did not say that to her other sister, so the only one who does not know about that relationship is Sakurako
Well, it's implied in chapter 11 that Hanako had seen her, althought it's not mentionned it's her GF, Hanako said that A friend is with Nadeshiko, meaning there is only one of her,implying it's her GF but Hanako's descriptions are vague (obviously) and Sakurako end up doing funny faceand make Hanako forget what she was about to say about the friend.