Nice to see this on the front page, it's been one of my favorites for quite some time.
To me that second-to-last page, with the message saying "sorry for stealing your big sister away from you" is doubly ironic--on one hand, that's the opposite of her problem, rather she feels like her big sister stole Kuga-san away, and on the other, Kuga-san (with her gentle, generous influence) if anything gave her back an older sister she hadn't had for a while.
Nice reflexion, Purple Library Guy. Kuga-san "giving back the older sister" went right over my head all this time...