Fella Pure: Mitarashi-san Chi no Jijou

Fella Pure: Mitarashi-san Chi no Jijou is a 2011 hentai manga.

The story revolves around siblings Keiko and Noboru Mitarai. Keiko is the president of the school council and is a very strict person. However, when nobody's looking, she keeps an incestuous relationship with her younger brother, though until the first chapter there was no penetration between them. In the first chapter, however, they finally confess to each other they feel more than carnal attraction to each other and engage on a genuine relationship.

Written and drawn by Fue. Serialized on Comic Milf.

Tropes used in Fella Pure: Mitarashi-san Chi no Jijou include:

Brother-Sister Incest: Keiko and Noboru, since they were kids, born from sexual curiosity. The relationship turns from purely sexual to affective as the story goes...But of course the sex doesn't stop.
Erotic Eating: Noboru feeding Keiko porridge while she's sick, especially since he decides to feed her with his tongue.
Failed a Spot Check: In chapter 6, there is an inversion: the siblings think nobody is seeing they're having sex while walking towards the bathroom, but some waiters clearly notice it but avoid bringing attention to it to not cause an scandal.
Fountain of Youth: The siblings receive special pills from their parents that turn them younger when digested. Keiko has fun with kid Noboru, and later we see Noboru "playing" with little girl Keiko.
Genre Shift: Chapter 4, where Noboru turns into a child, fits more Urban Fantasy than the Slice of Life of previous chapters.
Hair Decoration: Keiko wears a hair clip on the front of her hair.
Hypocrite: Keiko worries a lot about keeping the moral code in school but is violating a way worse moral rule-breaking at home: having sex with her own brother.
Locked Out of the Loop: Their parents obviously do not know they're having sex with each other, and Chapter 5 involves the two having sex through a hole in the wall while avoiding their mother's eye.
Love Confession: In chapter 1, they confess to each other, almost simultaneously.
Making Love in All the Wrong Places: Like in a restaurant and then a park. Keiko gets so excited by it that she decides that Noboru and she should have sex in a lot of public places from that point on in Chapter 6.
Sick Episode: Chapter 2 involves Keiko being sick and staying at home, and Noboru has to feed her.
Slice of Life: There is not really an ongoing plot, is just the two having sex under several situations.
Zettai Ryouiki: When they go to a date, Keiko uses a class S.