[quigonkenny]

I think what Kanon is noting as a spoiler isn't that the sister's joining the school, as that has been said in the anime, but instead something only apparent if you take a close look at her picture on that cover. (Depending upon how perceptive one has been so far, or how familiar one is with Japanese culture, some people may see it and still not get its significance) While I personally find such development patently obvious from logical progression and the premise of the story, it's not something that has been mentioned or suggested in the anime.