Given the scarring on the brother's (?) chest, probably some sort of post-apocalyptic scenario? Feels a bit like that one cute-girls-in-school SOL that turned out to be zombie survival horror.
Just a heads-up in case there's anyone new, this is the FIFTH arc of the Seishun Buta Yarou series. The story is progressive, and you'll be a confused by a lot of things if you don't read the series in order: