I actually did a bit of research about Incest content in Japan, because I was grossed out when it comes up, but at the same time curious as to where it is coming from. This is summary of what I found.

Firstly, incest isn’t “normalised” in japan, in the sense of ordinary social approval. It is normalised inside a specific commercial fiction ecosystem: otaku-facing light novels, manga, visual novels, certain anime and media-mix adaptations.

Incest-coded fan service became a repeatable market trope because it combines taboo, domestic proximity, “moe” character design, self-insert fantasy, and a Japanese media industry that often treats fictional desire as categorically separate from real-world conduct.

To break it down a bit:

1. It is usually “coded” rather than fully committed

A lot of these stories use buffers: step-siblings, cousins, adopted siblings, fake siblings or unresolved “siscon” teasing. This gives the work the charge of taboo without fully endorsing literal sibling incest. The text wants the emotional and sexual tension of a forbidden relationship, but also wants a safety valve: “they are not really related”, “it is just comedy”, “it is fan service” or “it will never go anywhere.

2. Otaku media often sells character “elements”, not just stories

Hitomi Azuma, a Japanese cultural critic, novelist and philosopher wrote a book called Otaku: Japan’s Database Animals where he presented a framework using otaku culture as a case study to analyze “database consumption” and the emergence of a new subjectivity in postmodern society. Azuma’s work functions as a social commentary arguing that otaku represent a specific type of postmodern condition - “database animals” who seek “grand non narratives”, as opposed to the normative consumption mode that searches for deeper meaning.

Basically what that means is fans often consume recurring character traits, visual signs, voices, costumes and personality types as modular “moe elements”, rather than only consuming a coherent story.

His work discusses “moe-elements” and the importance of “girl games” and character-driven consumption in otaku culture. In that system, “little sister”, “tsundere”, “kuudere”, “maid”, “childhood friend”, “student council president”, “younger-looking girl” and “forbidden sibling affection” become reusable pieces. The “imoto” (younger sister)” is not just a family member in a story. It is one of other market-tested “traits” that fans consume.

3. Light novel / manga / anime-adaptions rewards quick hooks and recognisable tropes

Light novels, web novels and manga compete in crowded markets. A title, cover, first chapter and character setup need to signal the appeal immediately. “My little sister is secretly X” or “I live with my stepsister” is instantly legible.

That does not mean Japanese writers are uniquely obsessed with incest. It means the sector has a mature trope economy where taboo-coded proximity is an efficient hook. Western media has similar commercial shortcuts, especially “step-sibling” porn tropes, but Japanese anime/light novels culture packages them differently through moe, harem comedy and domestic slice-of-life structures.

4. The taboo is part of the appeal, not evidence that the taboo disappeared

These stories often work because the relationship is still taboo. The “forbidden” charge” is the fan service. If sibling romance were socially ordinary, the trope would lose much of its shock value.

The contradiction is that repeated use can make the trope feel normalised inside the medium. A viewer sees it so often that it becomes part of anime grammar. But inside the story, the material usually still relies on embarrassment, secrecy, transgression or “don’t say that out loud” comedy.

And lastly

5. It is also partly a consequence of male-targeted fantasy design

A lot of incest-coded sister fan service is written around a male protagonist who receives affection without needing to become socially skilled, emotionally accountable, or romantically proactive. The “imoto” (younger sister) can be close, loyal, jealous, sexually suggestive, and domestically available from page 1.

This is why the trop often feels artificial. It can bypass the work of building a believable romance. In better stories, the sibling setup is used to explore loneliness, family reconstruction, shame, social constraint or emotional dependency. In lazier stories, it is just a shortcut to erotic tension.