Responses
You should read the previous mangas in this order: Allergy XXX, Secret XXX, Therapy Game. A few things might clear up!
They are talking about Secret xxx I think, this is a spin-off, the bottom is the brother of the top of Secret xxx :3 (I think lol :’D)
I've always thought the onii-chan is hot, but goddamn this spinoff made him 100x more better