Author: 日向夏 | Hyuuga, Natsu | Hyūga, Natsu | Twitter | Narou
Illustrator: 赤瓦もどむ | Akagawara, Modomu | Twitter
Kodansha Book Club entry for Volume 1
Bookwalker Series Page
MAL
ANN
Synopsis per MAL:
In modern day Japan, humans with "divine power" are called "Himiko," and once receiving national certification, they are called "Kamisama" or "God." After Nagi's grandmother passes away, the shrine in her hometown no longer has a god and is in trouble. Nagi's twin brother, Takeru, inherited divine power but unfortunately is a hikikomori or shut-in.
By chance, Nagi enrols in "Kamisama Gakkou," the school where Himiko attend. Compelled to revive the shrine, Nagi aims to receive national qualification and become a god.
My take on this:
Twin brother is the one with the proper prereqs but, well, hikkomori.
So either the enrollment notification was misdirected to Nagi because somehow they don't realize it's a set of twins or it was sent to Takeru who wasn't having anything to do with it and foisted it off on Nagi; that they are twins and the one going to the school is the one without the inherited ability is clearly key to the story setup, if nothing else.
Presuming that the synopsis from MAL is fairly accurate, that is.
"by chance" could well be "by mistake", as in the wrong sibling.
Something not mentioned in the synopsis is whether the school thinks the incoming student is male or female...
Interestingly enough, the illustrator for the LN is also the mangaka for the manga.
And, um, The Apothecary Diaries is published by J-Novel Club, and has considerable following. It is the same author.