I know its not translated (and its over a year since this page got a reply) but if you look at earlier pages, Chikuma had found a fairy and she eagerly showed it to her sister. Since her mother cannot see it (judging from her panicked face) the father comes back home from work and again judging from the pages and how the conversation goes, the father gets really pissed and decides to lock Tone away (who he deems not normal because again, the parents can't see the fairy) and because Tone decided to protect her sister and take the blame herself.
Its just like what they did with people they didn't deem "normal" back in the olden days. If the author ever comes back to this and does another chapter I'd love to see just how Tone gets rescued.