At one point in the Ah! My Goddess manga, a botched love potion fed to Keiichi causes any female to meet his eyes to become passionately infatuated with him — starting with his sister Megumi. She says, "Our love can transcend these foolish bonds of blood!" His horrified reaction: "Oh no it can't!" Fortunately, neutralizing the potion wipes her memory of the incident.
The anime-remade episode has this too, and is more outrageous, where Megumi goes to the length of clinging onto Keichii and fighting over him with Skuld, Urd and Peorth who also fell victim to the effect of the potion.
Minato and Juunichi's relationship in Akane-iro ni Somaru Sakais seen as this by everyone despite the siblings' insistence that it's not. It turns out everyone else was spot on, because when the show ends, Minato is the one who ends up winning. Interestingly enough, the level of incest between Minato and Juunichi changes depending on the version of the story. They're fully blood-related in the original PC game, but Minato is adopted in the PS2 version and possibly in the anime as well.
One of the main drives of Akatsuki no Aria's story is how Aria Kanbara and Natsuo Nishimikado are half-siblings, what with Aria being an Heroic Bastard, but are very attracted to each other. it turns out that NOPE, they aren't related, and later Aria has to search for her actual birth father.
Setsuna worries that this relationship will make Sara's life difficult, but their feelings are mutual. Even though both of them are consistently faced with opposition (whether from human society or from Heaven), their love never falters. They do, through the course of their separate adventures, mature and become aware of the sacrifices their allies are making to help them, and the impact their actions have on the people around them. In the end, they are reunited in Assiah (Earth), they go back to their human lives, still siblings, and Setsuna is sure they'll find a way to make it work.
The OVA ends much earlier, and with Sara dead and Setsuna killed so he may go to the afterlife to rescue her, but it abruptly ends right there, leaving it ambiguous if he'll succeed, but given they reference the story of Orpheus... high hopes are not had.
In Heaven, all close relationships are taboo, and all angels are related. But, with the exception of twins, they don't tend to think of each other as siblings. Though the dynamic is different, it still applies because if any of them have children together, genetic mutations are a valid concern.
The novel adaptation of Baccano! says that Graham Specter's first love was his sister. No information has been given beyond that so far.
Averted in Baka and Test: Summon the Beasts because while Akira Yoshii really wants to move her relationship with her younger brother to the next level, the normally super-horny Akihisa surprisingly expressly considers incest to be disturbingly immoral and constantly resists the nonstop attempts at seduction of his older sister.
Baku Ane: Otouto Shibocchau zo!: A guy's four sisters become his harem.
Battle Spirits Shonen Gekiha Dan has Yuuki and Kajitsu Momose. From the beginning, it seems apparent that their affection for each other goes beyond the norm for a brother and sister, especially given that this is a kids show. The truth is that the two were reincarnated lovers who unfortunately came back as brother and sister. Which didn't stop them from loving each other, as they were to be married in their previous life. Both die, and then re-incarnate without the blood relation. However, because they can't catch a break, this time they're stuck with Interspecies Romance.
Berserk subverts this somewhat, since Serpico and Farnese were in love at one point and are half-siblings, but Serpico's love becomes more brotherly after he learns that Farnese is his half sister.
Kyouka from B Gata H Kei is admittedly in love with her brother, wants him to take her virginity, and even has a Stalker Shrine devoted to him. He's oblivious. Eventually, when she gets a better understanding of what sex entails, she decides she never wants her brother to do something so dirty, and gives up on him.
Hansel and Gretel from Black Lagoon, even though it's not clear which of them is which, or if they're even different genders.
Bloods: Inraku no Ketsuzoku 2: The Sagemiya family is cursed in that its members are sexually aroused in the presence of another of the same bloodline. Misaki twin brother Shun are both disgusted over this and have sworn to resist the curse. However, Shun loses his composure and starts banging their big sister Kagu. After Misaki finds out, she too gives in to the curse and begins an affair with Shun.
Boys Empire is a shotacon doujinshi series that begins with Makoto regularly having sex with his older sister, Mina. When their mother, Umeko, finally realizes what they've been up to, she gets in on it, herself. They even work out a weekly schedule that determines which of them gets to sleep with Makoto on which days. By the end of the series, both of them (and his girlfriend, Hitomi) end up pregnant with his children. Umeko, in particular, gives birth to twins, with one being his father's child and the other being Makoto's, since they impregnated her on the same night.
Buso Renkin: Subverted with the Hayasaka twins who have quite a bit of Incest Subtext when they are first introduced, which is latter explained when their backstory reveals that they were kidnapped as children and their kidnapper/surrogate mother would have them play game built around wedding vows to build a strong bond between them. When their real parents rejected them, the two legitimately felt like they only had each other, and latched onto the one thing that was happy from their childhood. Once they realize that they're not alone, a lot of the creepy subtext gets toned down, and there are hints that Oka may have a crush on Kazuki.
In an unusually negative scene, there is a pseudo-incestuous rape sequence in Ceres, Celestial Legend. It also contains a subversion and plot device of this trope, seeing as Shisou Mikage, who is still madly in love with his wife Ceres, is reincarnated as Aki Mikage. Aki's twin sister Aya is the reincarnation of Ceres, and she is eventually led to kill him to save both herself and Ceres. Aki and Aya themselves do love each other greatly, but not in an incestuous way.
Candy Boy's main theme is the yuri subtext between (fraternal) twin sisters, Yukino and Kanade. The lyrics in the recap video appended to the first episode proclaim that "this may be the best pairing in the history of the world".
Due to technicalities regarding reincarnation in Cardcaptor Sakura, Syaoran is, technically speaking, Sakura's half-brother (although they don't know that until very late in the series). Sakura's father is a reincarnation of Clow Reed, albeit one without any known magical abilities, and Syaoran is one of Clow Reed's descendants, on his mother's side. This being a series by CLAMP, it doesn't really matter, or even factor much (if at all) into the plot.
In Chivalry of a Failed Knight, Ikki's younger sister Shizuku is interested in this. Ikki, to his credit, finds this highly disturbing and other characters note that her behavior is taboo. Her behavior is due to the neglect and abuse Ikki suffered at the hands of the Kurogane family, so she decides to love him enough to make up for every other member of their family. Shizuku's behavior can clearly cross the line of normal brother-sister relations, which causes quite a few raised eyebrows around campus.
Citrus chronicles the growing romance between two stepsisters.
Happens quite frequently (both within the story's present time and in flashbacks) between Michel and Coco in the manga Copernicus Breathing.
In Corsair, Jean-Hughes saves his seven year old brother Canale from being raped, but he can't stop imagining himself doing the exact same thing to him. This leads to him pinning the guilt and blame on Canale and loathing him even more.
Cain Hargreaves, the lead of the Count Cain manga and its sequel series Godchild, has this as part of his origin story. Turns out part of why he has the family curse so bad/ slash why his parents always acted so weird with him/why he has sort of golden eyes, is that his mother isn't his mother. His father's crazy sister Augusta is. When he finds out and goes to see her in her asylum, she jumps to her death in front of him.
For some reason, Kaori Yuki decided to reveal in the end that Magnificent Bastard Alexis was actually manipulated by his much more evil sister Augusta because... she was just that evil, apparently. Yeah, evil enough to seduce her brother into impregnating her and then spend the next twelve years catatonic in an asylum before killing herself just to traumatize the kid. According to her ghost, at least.
Dear Brother has Rei Asaka's obsession with her older half-sister Fukiko Ichinomiya, and Fukiko's abusive behavior towards her as one of the driving forces of the plot.
In D.Gray-Man, Komui's attitude toward his sister Lenalee straddles the line between Big Brother Instinct and this trope; even the other characters find it a little creepy and frequently remark on his obsession.
In Domestic Girlfriend the main character loses his virginity to his step-sister and is in love with his other step-sister. These all happened BEFORE they became a family.
Being focused on otaku it's no surprise this trope gets brought up and discussed in Don't Become an Otaku, Shinozaki-san! Makoto's otaku buddies are jealous the he has a real life little sister, but Makoto himself feels no attraction to her (least of all because she tends to treat him badly), nor to any of his sister's friends specifically because he's known many of them since they were very young. Akina has no interest either, viewing Makoto as a gross otaku even as she becomes one herself, and because she falls more under School Girl Lesbian anyway.
Don't Meddle with My Daughter!: In chapter 9, Artemis challenges her older sister, Athena, to a duel to decide whether she and her daughter would have to return to their home planet with her. The match is even, until Athena catches Artemis off-guard by suddenly tongue kissing her. Then she capitalizes on Artemis' shame and confusion by fingering her, and finishes her off by 69ing with her in midair. Artemis tries to retaliate, but she climaxes first. She concedes the match, but still calls Athena out for using such underhanded tactics.
Durarara!!: Namie is quite blatant to admit that her love for her younger brother Seiji is not familial. Even Izaya finds it a bit disgraceful.
In Yuki Suegutsu's infamous Eden no Hana manga, the protegonist, Midori, was put in foster care and then was raped by her adoptive brother.
All but expressly stated in Edens Bowy between tragic siblings Wietoo and Hairra. Initially averted when Hairra actually tried to pair her older brother Wietoo together with her best friend Fennis in an effort to repress her own incestuous feelings, but it failed anyway due to irreconcilable differences of ideology between Wietoo and Fennis, who also noticed Hairra's disturbingly excessive attachment to her sibling. Eventually the siblings get together when Hairra decides to acknowledge her feelings upon realizing that no one would love her brother as much as she did (which led to a falling-out with Fennis) and Wietoo also realizing that only Hairra would ever accept all of him, including his controversial views on the creation of sentient A.I. Hairra is soon killed tragically sometime afterwards when Wietoo's first android goes berzerk, and she is reborn as his personal gynoid weapon/bodyguard. Later, when they are both killed at the series end, they are reborn together as sentient programs together roaming forever their world's version of the internet.
In the Manga for Elfen Lied, this is the entire reason Lucy/Nyuu/Kaede's half-brother was born. Kakuzawa wants him to mate with Lucy/Nyuu/Kaede and procreate more Silphelits.
Eromanga Sensei from the same writer/illustrator team makes it clear that step siblings Masamune and Sagiri are attracted to each other almost from the outset. Masamune straight up declares his love in episode 3, though neither pursues it in part due to hang ups from their loss of family members - each is an orphan when the series begins, and he hadn't seen her in the 2 years since their parents died and she became a hikkikomori.
This is a plot point in Eternal Alice. The main characters are a boy named Aruto, his love interest Arisu, and his little sister with a very obvious crush on him, Kiraha. It is revealed later that Arisu isn't real, and Aruto made her up because he couldn't deal with his incestual feelings for Kiraha.
Ryo from Final Approach is quite obsessive over his little sister Akane and she's generally reciprocative. It's generally played for laughs as nothing more than an amusingly intense sister/brother complex, but it is worth noting that Akane insisted on taking part in the contest with the other girls to see who got to play the role of Ryo's bride.
When Akane is sick Ryo rushes home to take care of her. It's at this point the other girls realize they're not competing with each other but with Akane.
Averted in Fist of the North Star when Juuza discovers his childhood sweetheart Yuria is actually his half-sister. They drift apart soon after, having never consummated their relationship.
Averted in the final arc of Food Wars!. Asahi Saiba shows up out of nowhere and tries to force Erina Nakiri to marry him, much to her displeasure (and Soma's, for that matter). After Soma defeats Asahi in the BLUE, it's revealed that he's the son of Azami (Erina's father) from a one-night stand he had with a random woman when he was a student.
In Freezing, Satellizer was abused by her half brother. He molested her as well, though their blood relation was in fact the only thing that kept him from outright raping her, what he did was enough to emotionally scar Satellizer to the point that she could no longer stand being touched by anybody.
And, for some irony, the one guy who's the exception, the one guy who can touch her because he treats her with dignity and compassion? He was attracted to her initially due to her resemblance to his big sister, and and given they share genetic implants tied to said sister that give Satellizer a genetic resemblance, they are an indirect example of this trope as well, except Kazuya Aoi (the guy) is a far, far better choice for Satellizer by far.
Somethinged in Fresh Pretty Cure!. Miki's brother Kazuki lives with her father (her parents are separated, presumably divorced) and uses his name. She hasn't told her best friends they they're related, and in the first episode she refers to their upcoming outing as a date. The second episode focuses on one of their dates; it also features The Reveal that they're related, but you'd be forgiven for missing this detail. Eventually it's revealed that, when her friends assumed Kazuki was Miki's boyfriend, Miki went out of her way not to correct them because she thought it was cute.
Considering the yuri subtext in the Pretty Cure franchise in general, we have the only Precure-sisters Love and Setsuna from the same series. They are step-sisters, though.
From Up on Poppy Hill uses it as a Zig Zagged Trope when Umi and Shun fall for each other, find out they're related, resolve to continue the relationship anyway, then find out they're definitely not related. As far as they knew, they definitely fell into the trope for the few hours between deciding to continue and finding out the truth, though nothing physical happens during that time.
Renhou and his older sister Miiru in Fushigi Yuugi. In fact, their incestuous relationship got them killed by their people, and they go to the Big Bad Tenkou in hopes he'll help them stay together.
In Gankutsuou Andrea actually tries to marry and rape his half-sister Eugenie. Though only to deliberately humiliate her and her family.
Silvia in Genesis of Aquarion has a pretty obvious attraction to her older brother Sirius, becoming incredibly jealous of even the slightest hint of romantic interest in other women. It seems her insistent belief that he is the reincarnated lover of the woman she is the reincarnation of has a lot to do with this. In reality, reincarnation is largely to blame, but not in the way it first seems: they are actually both reincarnated from the same person, each holding a fragment of Celiane's soul. The longing of the two halves to be united again causes Silvia's apparent crush on her brother; he doesn't feel the same way because he got stuck with the negative aspects of Celiane's soul and memories, and so sealed them away at a young age.
In Georgie!, the Plucky Girl Georgie is romantically pursued by her two adoptive brothers, Abel and Arthur. Since the three were raised together and Georgie herself didn't know she was adopted for many years... well, yeah. In the anime, she doesn't end up with either of them but they leave together as a family. In the manga, however, it ends very differently. Georgie does fall for oldest sibling Abel. They even have the shoujo equivalent to a sex scene. By the end of the manga, Abel has been killed and Georgie has his child, and decides to return to Australia, where Arthur is waiting for her, having returned several years earlier.
In Ghost Hound, the main character, Tarou, has a slight obsession with his sister which leads to him accusing the girl he likes of being his sister reincarnated as an explanation for why he thinks about her all the time.
In Godannar, English pilot Knight Valentine will flirt with any female in sight, much to the consternation of Ellis, his sister and partner, who thinks he doesn't notice her. But if the way he looks at her swimsuit-clad form and the fact that he sees Ellis' face when an amnesiac Mila is hitting him with Charm Person are any indication, he does notice her in that way.
Happy World!: The explanation Takeshi uses to explain Elle's connection to him is that she's his step-sister. Cut to Takeshi exclaiming loudly to her in a classroom "We should make a baby." And they actually go halfway through the act before realising that they're not emotionally ready for it.
As it turns out, their relationship to eachother is technically step-siblings... probably. Takeshi's mother is Elle's mother, but Elle doesn't have a father, so it's a bit complicated.
Their child, Prayer, is eventually born... but not through normal means. Takeshi and Elle fuse their bodies, and Prayer is born when they split apart again.
Two examples of this, running in both directions, are present in Hatsukoi Limited. Ayumi's brother, Yuuji, is a definite siscon, and Koyoi Bessho is defined by her brother complex. Of course, they are but one small part of the Love Dodecahedron that permeates the series.
This issue is addressed in episode 9 of the second season of Hell Girl. The incestual love in this case is one-sided, and the offending sibling is, of course, sent to hell. The episode ends on a lighter note; the aforementioned sibling is not tortured on the ferry (unlike sinners who have done worse), and there is the knowledge that the brother and sister will eventually be reunited as per the terms of the curse.
Hetalia: Axis Powers has both the traditional brother/sister version and a Ho Yay one. On a hand, the Yandere Belarus would love to marry her older brother Russia (also a Yandere), which freaks him out no end. On the other, the Annoying Younger Sibling South Korea isn't much more successful in getting his older brother China (and sometimes his other older brother, Japan), to acknowledge his more-than-brotherly affection. Both cases are played for laughs.
Switzerland and Liechtenstein, while Not Blood Siblings are shown in-canon to be very close, leading to some Ship Tease. It probably doesn't help that they also act like a young married couple at times rather than as brother and sister.
Hime-Sama Love Life: An unpopular brother and his shy sister decide to have their first times with each other.
In His and Her Circumstances, Kazuma and Tsubasa simutanously get crushes on their other as their parents get married. Cue Sick and Wrong from parents, Wangst from Kazuma which make him run away from home, which makes poor Tsubasa down the mental road before it gets better.
Shinogu and Hatsumi from Hot Gimmick are Not Blood Siblings. Shinogu is in love with Hatsumi but, in an effort to not burden her with his feelings, he tries to put distance between them. Hatsumi actually offers herself sexually to Shinogu at one point but he turns her down. And in the Hot Gimmick S novel, she actually leaves her abusive boyfriend Ryoki for Shinogu.
Hybrid × Heart Magias Academy Ataraxia: Kizuna (age 17) has his first Climax Hybrid session with his biological 27-year old sister, Reiri, in volume 5 because there were no other options. However, a flashback to their childhood shows Reiri had an unnatural interest in her brother, even then and often bathed with him. They have additional sessions in volumes 8 and 10, by which time, Reiri fully accepts that she's in love with Kizuna. By volume 13, they have unprotected sex almost every day, usually for hours at a time. It eventually results in Reiri being the first to become pregnant with his child.
For the first volume of Ichiroh!, Nanako's brother had it bad for her and even followed her while she was off studying for exams. Nanako, of course, did not return these feelings. Eventually, he realized his sister was maturing while he was not and set off to improve himself to be worthy of her. We never see him again except in passing and omake strips, presumably because incest jokes in a light-hearts 4-koma strip would be kinda out of place.
Infinite Ryvius: We eventually discover that Ikumi had fallen in (that kind of) love with his dead older sister.
The Irregular at Magic High School has Miyuki Shiba, who is VERY attached to her older brother Tatsuya and has deliberately geared a good part of her life around him. On the other hand, Tatsuya certainly cares for Miyuki but he's got his own and huge issues and can barely feel actual emotions as a result. It turns out that Tatsuya's only truly remaining emotion is his brotherly love for Miyuki, but later, so as the body of his sister genetically "improved" is enough to nullify any effects of incest between siblings, their aunt announces Tatsuya her son and adopts him, so that they could marry as cousins. So later in the story, they're engaged to marry.
Kiss x Sis is about the ongoing incestuous affair between Keita and his older twin stepsisters, Ako and Riko, who take turns waking up him up each morning by making out with him beneath the covers. Their trysts become increasingly risque as the series (and their relationship) progresses, to where Keita has even brought Riko to orgasm by performing oral sex on her. Despite all this, they have yet to actually engage in intercourse, which remains in question as the manga is still ongoing.
Koi Kaze is a case of this trope being deconstructed and Played for Drama with two blood siblings who haven't seen each other since the brother was 18 and the sister was only 5 when their parents divorced and their mom took sole custody of the sister, and as a result, the Westermarck Effect didn't take hold. The series has a more harrowing portrayal and doesn't gloss over just how painful and frustrating such a situation would be for both parties involved.
Iina from Kokoro Library appears to have quite a thing for her sister Kokoro. She tries to steal kisses from her and gets quite upset when Kokoro decides to spend time in the bath with her other sister, Aruto, instead of her. Kokoro clearly loves Iina a lot too, but in a sisterly fashion.
In the spinoff of Genshiken, Kujibiki Unbalance has the main character's sister pursuing him endlessly, even going so far as to try perfume that is actually a love potion to seduce him. Instead she just succeeds in getting MC's best friend to kiss MC much to his sister's despair and anger.
The secret of the Kuhouin family in Kure-nai is that each girl born into the family is imprisoned in the "Inner Sanctuary" for their entire lives, for the sole purpose of bearing the children of their brothers to continue the lineage. Their very existence is kept hidden from the public, and the male members of the family marry women from outside the family just to keep up the masquerade.
Love 2 Quad: The hero gains power via ecchi activities, one of which is deflowering his younger sister.
Seimei is a Love Makes You Crazy version in Loveless towards Ritsuka. To Seimei, him and Ritsuka are the only two people in the world. The fact that the two are full-blooded brothers is probably the least squicky thing about him, though.
Magikano plays with the trope: The main character's oldest sister has a downright obsessive love for him (one that is definitely not simply sisterly) even to the point where she claims she will marry him and becomes ultra possessive whenever any other girl shows any interest. However he doesn't seem to have any interest in her. The arrival of the protagonist's Magical Girlfriend makes her intensely jealous and always vying for his attention. Despite the show's fanservicey roots, it's never portrayed as actually romantic or sexual though. The manga on the other hand...
In Mahou Shounen Majorian, Mari falls in love with the magical girl form of her brother, Masaru—which makes it kind of hard to classify.
In Marmalade Boy, Yuu and Miki become step-siblings when their parents marry (with both sets of parents splitting up and marrying each other), and then find themselves growing attracted to each other.Later in the series, they find evidence that they may be actually blood siblings; before they learn that this was all a big misunderstanding (said evidence wasn't referring to Yuu but to his mom's still-born child, who would've been his older sibling), they decide that they would marry regardless with the understanding that they will never have children.
Interestingly, the Mangaka (according to her notes on the series) originally planned for them to discover that they were blood siblings and split up for good, eventually hitching up with their unrequited love interests in a Distant Finale. Her producer convinced her that it wouldn't go down well with the fans...
In Maze Megaburst Space, not only is the central couple a pair of blood siblings, and not only is the central plot their successful and unrepentant plot to be together, but it goes even beyond that...
In Tsukihime, Shiki's sister Akiha has a very possessive crush on him. Parodied in numerous doujinshis and official Omakes, including one where Ciel points out the hopelessness of the crush. Shiki's feelings are purely platonic. And a large part sheer terror—Akiha is tsundere at best and yandere at worst. Of course, they areNot Blood Siblings, and Akiha does have her own path in the Visual Novel.
Though they never did anything incestuous, SHIKI hints many times that he has romantic interest in his sister Akiha. Too bad all she thinks about is her other "brother".
On that note, Azaka from The Garden of Sinners, who can be considered prototype of Akiha above, also loves her brother deeply, to point where it gets in the way of her becoming friends with the series heroine Ryogi Shiki. Her love may not be the same in nature as Akiha's but since she and her brother were used as a strong basis for Akiha and Shiki, it is likely that the "love for my brother" was carried over as well.
In One Thousand and One Nights, Dunya is infatuated with her older brother Sehara. She actually demands that he kiss at one point (which he does).
Oniichan Control has the brother who fled from his younger sister because he's developed inappropriate feelings for her. Unfortunately, his younger sister is not only fully aware of these feelings, but returns them in a big way. And she's a first-rate manipulator beneath her cutesy facade, managing to track him down, get admission to the same college he goes to, and worm her way into becoming his roommate. This goes so far that she's willing to stab a girl she thought was interestedin her brother. The entire series is about the brother trying to avoid this trope while the sister tries to invoke it.
In Ooku: The Inner Chambers, it's revealed that Akimoto and his sister Kinu slept together, resulting in a daughter. When Kinu asked if he entered the Ooku because he was disgusted by what they did, his response was that if he hadn't, he'd be forced to marry, and he didn't want to father a child with anyone but her. Emonnosuke is quite amused when he finds this out about Akimoto, and points out that even Japan's origins center around this trope. It's probably pretty telling that he actually gets one of the happier endings: after Shogun Tsunayoshi dies, he retires and returns home, meeting his daughter/niece for the first time, and is said to have lived the rest of his life out in peace with his family.
Oreimo: Whether or not siblings Kirino and Kyosuke do or don't have a sexual attraction/hidden desire to each other is the Elephant in the Living Room of the series. The anime adaptation leaves it hanging; the last chapter confirms it.
Ouke no Monshou has the Ancient Egyptian princess Isis who is in love with her brother, Pharaoh Memphis. Her jealousy towards any woman remotely interested in him extends beyond the grave, and it actually kicks off the plot since Carol, The Heroine, fell victim to one of Isis's curses and then was thrown into the Egypt of the past...
Penguindrum's first episode has Kanba Takakura kissing his younger sister Himari after she's resurrected and while she's sleeping. It hasn't been referred to later, though. Let's give it time.
It's given a rather philosophical twist in this series; Kanba's Internal Monologue in the scene revolves around defying fate and the animalistic survival strategy to determine one's free will. Basically, doing something that your very biology abhors is used as a sign for freedom of choice.
In the Penguindrum novel, the scene where a posssessed Himari pulls something out of Kanba's chest ( his life energy) is a little more suggestive than in the anime version. Not to mention, in the anime the kiss scene went into fade to black when Kanba's lips were milimeters away from Himari's. In the novel? It's described quite in detail.
Aaaand then... it's subverted! Himari and Kanba are Not Blood Siblings. Episode 19 explains that Himari was adopted into the Takakura family after she was rescued from a plance named the Child Broiler... and her savior was her and Kanba's brother, Shouma. Kanba himself is adopted too; he and Shouma relied on each other to survive, but he only felt truly as a Takakura when Himari welcomed him into the family.
Then finally reintroduced in the form of Twincest when Masako, Kanba's stalker and apparently jealous ex, turns out to be his biological twin sister, and Masako and Kanba were both well-aware of their relationship the entire time. Having physically assaulted and mind-wiped the rest of Kanba's ex girlfriends since she didn't see them as good enough for him, Masako ultimately turns her sights on Himari, whom she sees as both familial and romantic competition, but later sets it aside when Kaba pulls a FaceHeel Turn.
Please Twins! has what may or may not be an incestuous Love Triangle, between a boy and two girls who both claim to be his twin sister. To add to the blurring of the line, the girl who turns out to actually be his twin is different between the anime and the novelization. Interestingly, though, the setup of the plot clearly steers away from this: whoever turns out to be the twin must step out.
That said, both girls kiss Maiku before it's over, so be ready for some Squick. The possibility of being his sister keeps them from getting really aggressive, but that's all it does; their ambitions are not deterred.
Meanwhile, perhaps to provide contrast, there's a minor character (Matagu) who's blatantly attracted to his little sister Haruko.
And Haruko doesn't quite reciprocate, but at least clearly knows and doesn't mind. After describing his use of the Hands-On Approach, we get this:
Haruko's friends, in unison: "Your brother's a pervert!"
Haruko, cheerfully: "He is!"
The attraction in this series isn't all that unrealistic. The Westermarck Effect isn't in play, since the twins were separated at a very young age (maybe 2 at the latest), and the general principle of genetic attraction makes it unsurprising that the sister would have some feelings upon seeing someone who may (not) be her brother. That the other is a childhood friend makes it all the more probable, since they would have subconscious affection for one another at the outset. There is some kissing and some hand-holding/hugging between the would-be sister and the brother. Otherwise, this is an Averted Trope, since, as stated, the twin is not allowed to continue the relationship (which is part of the drama: which girl has to give up her romantic affections, and which girl gets to be by his side albeit as a sister, not a lover).
Maiku does have a hard time not having sex with either of them in the Beach Episode (complete with imagining them naked, though admittedly chibified) and has to resort to leaving the room to contain his libido. Knowing one is definitely his sister doesn't stop him from wanting them BOTH, although in fairness he doesn't know which one, nor does he act on it.
Ultimately, the characters are fully aware that such a relationship is ultimately not acceptable, to the point that both girls hope that they aren't the sister, possibly allowing a kindling of a relationship. Regardless of which one turns out to be the sister based on medium, the story presents this as a tragic thing, and said girl does stop the relationship in awareness of the gravity of the situation.
She wants to with sleep with Leina and pouted when she wasn't allowed to spend the night in her room, in the first episode. Leina is more than aware of Elina's blatant incestuous crush on her, which is part of the reason she ran away from home and doesn't intend to return. Though she still indulges Elina on occasion. Such as Episode 9, where she bathed together with her, then spent the night spooning with her in bed, while both were in the nude.
It's also implied in the main series that Claudette is in love with Elina, but her feelings are unrequited due to Elina being in love with Leina.
The "Hide & Seek" manga continuity took the subtext and ran with it. While Elina's feelings for Leina remain paramount, the manga features a romantic subplot between her and Claudeete and even has Leina say she supports her sisters being together (Chapter 28).
In Revolutionary Girl Utena, not only does Touga's sister Nanami harbor an intense crush on her older brother, but she also has a near-insane jealousy of anyone or anything that might attract more of his attention and love than she does. There is also the strange love-hate relationship between Kozue Kaoru and her twin brother Miki, best displayed in the bizarre bathtub-and-straight-razor scene from the Utena movie. Most important to the plot, though, is the semi-consensual relationship between Anthy and her manipulative and domineering brother Akio. In the first example, a few of the last episodes of the series deconstructs Nanami's infatuation with her brother by introducing the possibility that they are not siblings and then reveals that they are siblings; Touga is just being manipulative and explores Nanami's emotional confusion resulting from that distancing.
Ali and En in the anime-only "Makaiju" ("Doom Tree") arc of Sailor Moon. This is explainable to an extent, however: not only are they the only surviving members of their species, but they are "children" of a tree, which used what remained of its powers to "grow" them after the rest of the species was destroyed in warfare; thus making them "siblings" in roughly the same sense as Adam and Eve.
Lampshaded once by Makoto, when "Natsumi"/En bitches "Seijuurou"/Ail out for letting Mako offer him a part of her lunch. She's squicked and says something by the lines of "Wow... these siblings! So weird!"
DiC, perhaps as bowdlerization, had Crimson Rubeus refer to his four main female minions as sisters. He may have been referring to their name as a group, the Phantom Sisters, but it's suspicious when you realize most of them are attracted to him.
Saint Beast: Definitely played with in Kira and Maya, particularly in the official art.
In the anime of Samurai Deeper Kyo , Big Bad Nobunaga Oda and his right-hand henchwench Shatora are an incestuous brother-sister pair who desire who turn the "boring world" of Sengoku-Era Japan into a literal "hell of battles" so they can openly be together in the chaotic new world. They reveal that their Start of Darkness was because society tried to force them apart by making their relationship taboo in the first place.
It is heavily implied in the Shadow Star anime that the bully Aki Honda and her older brother Yasuhito have a ... rather intense relationship. It's downright spelled out in the manga. So much that Aki is once shown straddling Yasuhito while practically naked. Then, Oni shows up, kills Yasuhito by smashing his head on the wall, and rapes and kills Aki as revenge for what they did to his master, Hiroko. Said scene is censored in many countries in the world (even the Japanese version got censored retroactively) and spelled the end of the series' publication in many others.
Kanan is Hakkai's older sister in the Saiyuki manga, although this was edited out of the anime version, and, as they had been Separated at Birth or very close to it, they weren't originally aware of it. A tie-in novel also suggested that they might be Twins as well, which was approved by Word of God.
15-year-old Sakurako Saiki from Sakura Gari is a scary Yandere who is incredibly possessive of her older brother, the main character Souma, and has had sex with him more than once.. For further subversion, Sakurako is actually a male, Youya, who calls himself like that after the horribly fucked-up death of his mother... whose name was Sakurako.. And even more subverted: They're Not Blood Siblings. Souma is the son of the Saiki leader and his dead lost love, Abigail; Youya/Sakurako is the child of the original Sakurako and the retainer Katou. When Youya/Sakurako commits suicide, Katou reveals this and stabs Souma - apparently, to death.
Deconstructed to the point of Fan Disservice in Sasami-san@Ganbaranai: In order to keep the power of Amaterasu in their bloodline, siblings in Sasami's family are forced to marry. Due in part to what is apparently a lobotomy, the sibling which does not possess Amaterasu's power is conditioned to care for and unconditionally love the one that does.
Tiriel and Sorath in Shakugan no Shana have an incestuous relationship, though in this case it's portrayed in a creepy light. What theyare muddles the water, but only somewhat. Shana seems to take more offense at the French kissing than anything else.
An example also occurs in Yuu Watase's pre-Fushigi Yuugi manga, Shinshunki Miman Okotowari. Asuka Higuchi falls for her half-brother Manato Sudou early on in the story, and he eventually comes to reciprocate. And then they learn that they aren't blood-related after all—in a very mind-twisting way.
Shoujo Sect: Innocent Lovers: In the first episode, Chizuru goes to Naitou for advice on how to confess her feelings for the girl she was secretly in love with. Since Naitou didn't feel like being bothered, she tells her to just say so and get it over with. Adding that she didn't want to hear anymore about it until Chizuru had finally done it, and demanded she provide proof. So she confesses her love to Shigure that night. Just when it seemed that her feelings were unrequited, Shigure reveals she'd secretly been in love with her too and makes love to her. The next day at school, Chizuru delivers on her promise by showing Naitou a photo of them in bed together, which she had taken with her cellphone camera. When Naitou sees the other girl had been Chizuru's older sister, she regretted having said anything since, had she known, she would've advised against it.
Hoshina Utau from Shugo Chara!loves the resident Bishōnen catboy of the series, Tsukiyomi Ikuto. The thing is, her real family name is Tsukiyomi and she is Ikuto's sister, which does explain why Ikuto is so cold towards her advances — in fact, he's downright disgusted when she kisses him
Alti in Simoun is obsessed with her sister Kaimu, and tries to prod her into choosing to be female so that Alti can become male and "protect" her.
They also have sex once during the course of the show (long story) and in the Distant Finale they are living together even though they both chose to remain female.
Sister Princess and its "sequel" series, Sister Princess Repure, both have incestuous subtexts, but the subtext surfaces clearly in Repure: in the very first episode, Karen recounts how she decided that she would be her brother's bride, despite society's disapproval of such a thing. In the original series it is less visible, although still present — one sister (Chikage) was Wataru's lover in a previous life, Sakuya openly flirts with him (Rin Rin and Mamoru are more subtle hiding their flirting under the disguise of admiration for their Brother), and all twelve spend one episode making mock wedding dresses so they can play at being his "bride".
Aono in Sola is obsessed with her (younger, for a change) brother Yorito to the point of creating a copy of him that took enough power to leave her bed-ridden for years.
In The SoulTaker, Runa wanted to marry her twin brother Kyōsuke, who declined the honor.
The Spiral manga has a variation: Ayumu was in love with his sister-in-law even before his brother married her, and lived with her after he disappeared. Also, since all of the Blade Children have the same father, Kousuke and Ryoko's relationship, despite growing up as childhood friends rather than siblings, is incestuous. Kousuke specifically asks Ryoko not to call him 'big brother,' saying that he wants to be able to dream of being with her.
Swing Out Sisters: Yuuta's two older sisters have the hots for him, and he starts intense sexual affairs with both. The sisters agree to share their brother, and find that they enjoy each other's bodies as well, thus forming a stable sibling love triangle.
The original OAV series of Tenchi Muyo! is filled with incestual attractions — the whole mess basically got started when Ayeka fell in love with and was engaged to marry her elder half-brother Katsuhito/Yosho. See Incest Is Relative for full details.
Ren and Mihato in They Are My Noble Masters have a mutual brocon-siscon relationship, and are quite supportive of each other. It's almost sweet if one is willing to look past the sibling thing. Then again, this is a show that's filled to the brim with fetishes. They do have a route together in the original h-game.
In the same series, Shinra is very open about her sexual attraction towards Miyu, her younger sister. Worse, she sexually assaults her on numerous occasions, which is played entirely for laughs. That Miyu seems to be very ambiguous about this and that the youngest sister is actually jealous for being left out just adds to the squick-factor. Fetishes, indeed.
In Tokyo Ghoul :re, Nimura Furuta states that he has been in love with Rize since they were children, and intends to marry her once his plans come to fruition. In fact, his decision to nearly kill her and allow Dr. Kanou to imprison and experiment on her was because he couldn't stand the thought of her having children with anyone else. It becomes even creepier when one examines the family tree in the background, because Tsuneyoshi Washuu fathered all of the children in that generation. That makes Rize his half-sister, in a family that routinely practices Royal Inbreeding.
To Love-Ru seems to heading this way, if Momo gets her way. Played for laughs when Mikan trying to resist the effects of Celine's pollen.
She seemed more annoyed at his using accompanying Nanoha to visit Suzuka as an excuse to see Shinobu, given that he doesn't seem completely open about his relationship (In A's episode 9, she teases him about being able to work with Shinobu for Christmas with a cute smile on her face). Interestingly enough, Miyuki calls him "Kyou-chan" (like she did in Triangle Heart) instead of "Onii-chan", like Nanoha does, which may indicate that they're Not Blood Siblings.
In True Tears what starts as a case of Big Brother Instinct evolves into borderline incest when we learn that Jun has romantic feelings toward Noe, his younger sister. This trope is also averted at one point Shinchiro and Hiromi are under the impression that they are siblings but it turns out that they are not related.
It's revealed at the end of the oneshot manga Up And Down by Nishi Uko that the unnamed woman the protagonist pines for was in love with her sister. Her sister died so she adopted her niece. The woman states that she isn't sure if it's a blessing or if it's some sort of divine punishment because her niece looks a lot like her mother.
In Urusei Yatsura, the Mizunokoji siblings have a one-sided version of this going on... more or less. According to their screwed up traditions, youngest daughter Asuka was raised in total isolation from the opposite sex, and with the terrible job her maids did in regards explaining the other sex (something worsened by her fact her first encounter with a guy was Ataru Moroboshi), she's terrified of men... except for "Big Brothers", whom her maids made out as a special, dependable, trustworthy exception to men in general. Between her phobia and her naive ignorance, she not only has a crush on her older brother Tobimaro, but is constantly trying to perform deeds with him that are only appropriate between lovers. Tobimaro, meanwhile, is perfectly aware of their relationship and that the things she tries are not appropriate, thus he freaks out when the poor girl comes on him. The fact that their crazy mother blames Asuka's actions on him, despite the fact she was the one who arranged for Asuka to be raised the way she was, only makes things more galling for him. The fact Asuka also has Super Strength and is thus prone to crushing his bones whenever she gets frightened or romantic does not help.
Vampire Knight: Yuki, Kaname's adopted sister, becomes involved in a romantic relationship with him; this wouldn't normally count, except for The Reveal that they are biologically related and this doesn't stop the romance. Even more so, pureblood vampires like them actually encourage incestuous relationships, and Yuuki's parents Haruka and Juri were siblings too. (And their brother Ridou wanted Juri as his wife as well, which causes him to fixate on Yuuki...)
Lola and Layla from Venus Versus Virus have implications of a Sister-Sister version of this. Lola seems to not recirpocate her obsessed twin sisters feelings though.
In the first episode of the Voltage Fighter Gowcaizer OVA, the Villains Of The Week were an incestuous set of twins (who were empowered by the Big Bad to save the dying sister) who merged into a gestalt monster "powered by love".
Near the end of the first episode, Astral stumbles into Maria's cell and loses consciousness. When he awakes moments later and sees her state of undressnote due to Stallion's attempt to rape her, mere moments prior, he decides he likes what he sees and rapes her. Though it isn't explored in the OVA adaptation, the source material heavily implies that Maria and Astral are the biological offspring of Pollux and his wife Maria - who bequeathed her ring to her daughter of the same name. Meaning, Astral raped his biological sister without realizing it.
Following the timeskip, Maria now has an 18 year old daughter named Miyu, who's heavily implied to be the result of Astral having raped her. Astralnote who's amnesiac by this time, due to taking the full brunt of Maria's MystralWindow, which propelled him 20 years into the future and left him without his memory meets Miyu after saving her from a rampaging bear, while she was bathing in the forest, and ends up sleeping with her. That's right: he banged his own daughter/niece... without being aware of it. Again. Talk about keepin' itIN the family.
Pretty much the fundamental appeal of Yosuga no Sora combining with Twincest. Both tropes are also deconstructed: Soras attraction to Haruko is due to her physical and emotional dependence on him, coupled with her psychological instability, and while Harukos feelings are mutual, he struggles with the morality of their relationship, knowing society wont accept them. Their arc ends with the two moving overseas, hoping to finally live happily together.
A Surprise Incest variation appears in Yuureitou. Like just about everyone, Satoko is attracted to Tetsuo. As it turns out, Tetsuo is Marube's son. This makes them siblings. It never goes anywhere as the feelings are unrequited. It also may be a Not Blood Related situation because it's heavily implied that she is adopted.