Yeah, I was going to ask if she was the one who ran away to follow her idol. At least now they have someone to put on a body since the idol ghost is not fond of it.
And I'll say that Hiroto and Nirasawa are one of the cutest couple around this media (and I'm aware of the content of this manga). They really go so well together that I doubt that even a "reformed/back to her senses/good" step sister would stand a chance. Unless we're really going for the theory that this is not the original Hiroto from the last chapter.