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The Little Sister Heroine is a trope common in multiple Japanese media that combines the idea that little sisters are cute and that cute things are to be fetishized. It goes like this:

Little sisters are cute, right? And cute girls draw in viewers. And cute can be sexy. Take a few more 'logical' steps down the lines, and suddenly little sisters are love interests in anime, video games and visual novels. It is in fact one of the most popular heroines or love interests in Visual Novels, branching out from there into anime, perhaps surpassed only by the ever-present tsundere and childhood friend. Like Childhood Friend Romance, it is extremely prone to lampshade hangings.

As a Love Interest trope based on the little sister/older brother dynamic, the story will usually dwell on the romantic tension a fair amount, though as always the brother may be completely oblivious. Despite the name, Little Sister Heroine attempts to mitigate the obvious incestuous fallout; They�re usually either a step sister, adopted or, somewhat more rarely, technically a cousin or other more distant relation. In almost all cases, the younger sister is going to be either some degree of tsundere, childish and clingy or simply highly devoted to their older brother. Finally, the relationship between the two will likely be noticed or exaggerated by the rest of the cast. If you don't notice at least several of these general characteristics, it's probably not this trope.

It rarely works like this in real life. Those who have younger sisters themselves and hear about this trope often claim that they're more frequently annoying than cute, making this trope more wish fulfillment than anything like an accurate portrayal of sibling dynamics. Thanks to the Westermarck effect

it is far more likely that the idea of any kind of romance with your sister will be either laughable or even outright disgusting.

Also known in some circles as an imouto heroine.

See also Brother-Sister Incest and Flirty Stepsiblings. May overlap with Big Brother Attraction and usually will with Not Blood Related.

Examples:

Anime

The premise of Ore No Imouto Ga Konna Ni Kawaii Wake Ga Nai is largely based around this trope. Kirino plays games with this trope as their focus and is the tsundere type herself. The relationship between her and Kyousuke is probably the biggest focus of the series.
The World God Only Knows has Keima absolutely refuse to acknowledge that Elsie is a proper little sister because she doesn�t match the template from his beloved gal games. Eventually, however, he decides to acknowledge her, but only because he wants to get their contract completed quickly. As time goes on, she eventually really does become his little sister, moving away from this trope in the process.
Kara no Kyoukai has Kokutou�s little sister Azaka. She has an inherent interest and draw towards things that are taboo, so she develops an incestuous crush on her brother. Azaka is the template for Akiha in Tsukihime.
Yosuga no Sora has Sora, the main character's twin little sister. She has very strong feelings for her brother, going as far as thinking of him during... certain private times.
- A justification is attempted by showing, in a flashback, that Sora spent most of her childhood hospitalized, and so Sora and Haruka didn't spend enough time together for the Westermarck effect to kick in. Why Haruka was kept away from his sister for so long is never explained.
Kureha of Mayo Chiki! is a member of Jirou's Unwanted and Supporting Harem. Observe her reaction upon finding that Jirou, who has gynophobia*
That is, having a bad physical reaction to even being touched by women, which is largely Kureha's fault in the first place because of her treatment of him
may have a girl over at the house. It's not ignoring something that's none of her business, or happiness that her brother has finally gotten over the condition that has crippled his social life, but rather to kick down doors and scream:
Kureha: Who's been swindling my brother!?
Parodied in Dokkoida, where the main character lives with an alien girl masquerading as his little sister. Thanks to a soap opera episode following this trope, she and the majority of their neighbors (who are also aliens) become convinced that it applies to the main character as well, thanks in part to Dirty Old Man Dr. Marronflower using his own library of H-games as evidence. Hilarity Ensues when the two who have a crush on the hero attempt to invoke it to their advantage, leaving him completely confused.
Nao from Oniichan No Koto Nanka Zenzen Suki Janain Dakara Ne, whose life goal is to hook up with her adopted brother Shuusuke.
Taken to a whole new (hilarious) level in Nisemonogatari with Araragi's little sisters, Karen and Tsukihi. That infamous toothbrushing scene.

Video Games

Eien no Aselia has Kaori, but in the end she turns out to be more of a Living MacGuffin than a heroine and does not even have a proper ending. Yuuto is frequently accused of being a siscon due to his overprotective nature, but in the end it turns out to work in reverse due to her unrequited affections.
Men at Work 2 has Mena as a possible love interest, though not the main one.
Persona 4 has your cousin Nanako, who refers to the Protagonist as "Big Bro," gain a crush on him at the end of her social link. Nothing gets made of it, unless you also max out her father's social link, in which case his only protest is that Nanako is still too young.
Parodied with Desco in Disgaea 4: A Promise Unforgotten. Desco was created specifically to fulfill Fuka's childhood wish for a little sister, and as such was implanted with all of the classic traits of a Little Sister Heroine. However, since Fuka is an eccentric individual who wished for her Little Sister Heroine to come with "advanced functioning to help me take over the world," Desco was also created with Lovecraftian Superpowers and a compulsion for complete annihilation. The result is the world's most adorable Eldritch Abomination.
Ar Tonelico 2: Coccona is Croix's adoptive sister and very cute. She confides in Cloche that she is in love with him but his only love is as a brother because of their age difference. she's the default love interest.
Mia in Duel Savior Destiny is definitely the Clingy Jealous Girl type with some level of tsuntsun going on. Her actual degree of relation to Taiga gets retconned, though. In the epilogue of her route, it's hastily claimed that it sure is convenient that they're not related. In the final route, it turns out they actually do share a blood relation and are probably half siblings.

Visual Novels

Akiha in Tsukihime is largely written around this. Unfortunately for her, except for in her own ending this means she never graduates from �little sister�, even in sequels and expanded universe material. A definite tsundere type, she's even called Imouto (Little Sister) by Arcueid rather than her actual name.
Nemu in Da Capo, where she�s even the main heroine. In a rare case of not being completely Oblivious to Love, Junichi is in fact perfectly aware of how she feels but isn't quite comfortable with a relationship upgrade. Nemu is quite the tsundere and very clingy.
Da Capo II actually has two adoptive sister figures, but the one who plays it up more is Yume, who is younger than Yoshiyuki. Yume, a clear Nemu expy, is in general less clingy. Everyone at their school seems to be jealous of Yoshiyuki having such a cute little sister despite his frequent claims that there's nothing between them.
Canvas 2 has Ellis, though she�s actually his cousin. She tends to be extremely clingy and childish.
G Senjou No Maou has Kanon. Not only is she a heroine, but Kyousuke appears to be the only person she actually trusts. Kyousuke himself lampshades the romantic overtones of the trope, but doesn't appear to take it seriously until she confronts him about it halfway through the main storyline, at which point her own miniroute splits off.
Suika subverts the general trend of the little sister being a heroine when Akane stabs her stepbrother to death should he choose her. She�s actually in love with his fianc�e, so choosing her means that Akane feels that it's okay to take his place and brainwashes his fianc�e accordingly.
In Family Project, Matsuri eventually moves in to a little sister position for Tsukasa, even adopting the Japanese Sibling Terminology. Her route, however, tends to focus on the fact that she's far younger than he is.
Kara no Shoujo has Yukari, but despite the Brother-Sister Incest all over the story, you cannot sleep with her. The very idea clearly would not occur to Reiji.
- She does have a topless CG though.
Fate Hollow Ataraxia as a general sequel to Fate/stay night that contains all of its continuity has open acknowledgement of Ilya as Shirou�s little sister, a relationship set up in several endings. And she really exploits it too: She's clingy to the point of nearly picking up her former yandere traits and tends to act her apparent age rather than her real age. She still gets no adult scenes, though, despite being the closest thing the series has to a fourth heroine.
Yume Miru Kusuri has Aya, but her route was cut. It is strongly implied that she fell in love with Kouhei because he was always subtly out of place in their home, making it difficult for her to see him as her brother.
- Like the The World God Only Knows example above, Kouhei appears to be frustrated with her at one point for not being anything like the little sisters in H-games. "You're supposed to all me onii-san!" Aya is appropriately confused.
Chaos;Head has Takumi and his little sister, Nanami. The way the game works means that as with the other female main characters, you can trigger certain 'delusion' events about her that occur only in Takumi's head, but that he doesn't control. Or something. Though it turns out that he's not Takumi. Exactly. It's complicated.
Lampshaded with Rizu in A Profile.
Masayuki: I�m not marrying you. I want to make sure you don�t fall for me or anything. We�re only stepsiblings, so I thought I should make that clear.
Kana Little Sister has the main character's terminally ill little sister as the main heroine. The VN itself is possibly the Trope Codifier for this kind of relationship.
Inverted in Crescendo, where the older sister (Ayame Sasaki) is the one who gets the path. And yes, she's very pretty too.

Real Life

In the closing stages of World War II, Imperial Japan created the Chiran Nadeshiko (Ladies of Chiran) Units. Middle-school girls, they served as surrogate 'little sisters' for the Tokubetsu Kogeki*
literally 'Special Attack'
pilots receiving their somewhat brutal training at Chiran Airbase in Kagoshima Prefecture. The unit was highly publicized at the time and may even be the Trope Codifier. Growing close to so many (young) men sent off to die in 'Kamikaze' attacks took its toll, and many if not most of them subsequently committed suicide.

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