Kyoudai Shikkaku (HGT Labo)

A very normal sister and her younger brother. The only thing that's not normal is that the younger brother wears his older sister's school uniform. While they're having a silly conversation, the older sister puts on her old school uniform as well. When looking at his sister who's different from usual, his heart starts pounding and he's gradually unable to hide it, and finally the mood turns into a questionable atmosphere.. (Source: *An improved version with a different cover was released, which has extra pages and additional/changed sfx in some scenes, but the story is mostly the same.*