Brother-Sister Incest/Fire Emblem

This is a regular recurring theme in the Fire Emblem series. Not quite as much as the fandom believes, but several of the female characters have strong feelings towards their brothers, and one or two boys have similar feelings about their sisters.

Jugdral

• In Fire Emblem: Genealogy of the Holy War, Princess Lachesis and Lord Eldigan of Nodion (she even straightforwardly says she won't marry a guy who's less cool than her brother). It's hinted that Eldigan still considers Lachesis his Most Important Person, even if he does love his wife Grahnye. Sigurd says in chapter 2 that he cares about his sister more than anything, and Lachesis's name is on his lips just before he dies.
  • The Oosawa manga makes this even more blatant, with them acknowledging in their last talk that they're in love ( with Lache almost asking Elto to be her first, but he refuses to not taint her, settling for an implied Last Kiss) and hoping there's a Reincarnation Romance deal out there... but sadly demonizes Grahnye by making her a Clingy Jealous Girl who only allows Lachesis to join Sigurd's trope in open hopes to get her killed in battle and is cold and indifferent to Elto's death unless it makes Lache cry, rather than portraying her as simply jealous and lonely as would be expected. (And let's face it, it'd be weirder if she was not jealous.)
  • The Fire Emblem Heroes Tempest Trial event "Dancing Affinity" amps up Lachesis's feelings for Eldigan by having her refuse to dance with anyone who isn't her brother, and though Eldigan isn't quite as receptive he obliges her anyway. Plus, there's this gem from the ending:

"Even alone with me, his mind is ever somewhere else, with someONE else. I can never have him all to mysel—"

• • Cousins Ares (son of Eldigan) and Nanna (daughter of Lachesis) can be predestined and have a special love dialogue which directly discusses the trope as it alludes to Eldi and Lache's past.
• Deirdre and Lord Arvis. In this case, having the two hook up and breed is actually a part of the Big Bad Manfloy's plan to create a vessel for his Demon Lord. And through Mind Control in Deirdre's part and Arvis not knowing they're half-siblings, it happens.
  • According to the designer notes, Alvis does know. Thing is, he learned that Deirdre was his sister after they got married and had kids. By that point, however, Arvis's life sucked so much that he decided to not care anymore, he still loved Deirdre and nothing woild change it at that point.
  • The Oosawa manga twists things around a bit: Deirdre is the one who found out after their marriage, but she kept it to herself for similar reasons, and only told Arvis when she was about to die.

• High Priest Claud and the Dancer Silvia, whom many fans believe are really brother and sister. They both possess the Blagi bloodline, and Claud once mentions that Silvia reminds him of his sister, who disappeared many years ago (and Silvia says she's an orphan). And yes, they can potentially marry each other and have children (who are playable characters in the second half of the game), with the pairing being considered the "most canon" for them both. Though an official book shows that they're distant cousins.
• Tailtiu's daughter Tine may have these kinds of feelings for her brother Arthur:

Without Lovers Seliph: Tine, you’ll be going with Arthur too, aren’t you? Tine : I, I want to be of some help to my brother, that’s all. Seliph : You and Arthur are really close to each other, eh? Tine : That’s because...I like him... Lover Dead Seliph : Tine, you’ll be going with Arthur too, aren’t you? Have you forgotten [character] already? Tine : Ummm, no, I don’t think I ever would, but I’ll just try to do my best at going on with my life. I’ve met a very wonderful brother after all, we’ll be trying our hardest together.* (The translator is TRYING HARD not to make it sound like incest)

• • Seliph and Julia are half-siblings, the former being the son of Sigurd and Deirdre and the latter the daughter of Deirdre and Arvis. Their feelings are left rather ambiguous, but Seliph is very protective of her and offers to take her shopping to make her smile when she's stressed out (this is before they even find out they are siblings). However, the game data is specifically designed to reduce their Relationship Values over time. . . unless the player exploits a certain glitch or keeps them glued to each other. . .
    • Seliph's other Love Interests seem to feel bad for Julia, especially Lana if she and Seliph engage in their "Lover Conversation" on the final chapter.
  • We also have Arion and Altena, the biological son and, er, "adopted" daughter of King Trabant of Thracia. And by "adopted", we mean Trabant killed her parents when she was 3 years old and took her as part of the spoils of war, so he had a member of the Noba bloodline to wield the legendary weapon her father had, the lance Gaebolg.
    • Altena and Arion are descendants of the Crusaders Njorun and Dain respectively, who were brother and sister (though thankfully, not married to one another; Njorun's husband was Dain's Number Two). So they're technically Kissing Cousins, albeit very distant ones. Plus Altena can't fall in love with anyone in your army due to her feelings for Areone and she alone can recruit the guy into an allied unit for your group.
  • Special mention needs to go to the ability to pair up Lana and Lester with their cousins Febail and Patty. Their mothers are sisters, so from a social standpoint they're cousins. However, their mothers are actually Edain and Brigid... who actually are identical twins, which means that from a genetic standpoint, they're half-siblings. It takes some skill to make two couples simultaneously fall under Brother-Sister Incest and Kissing Cousins. Oh yeah, both couples are also predestined and very easy to get.

Elibe

• Fire Emblem: The Binding Blade: Clarine of Reglay has a bordering-on-crush admiration on her brother Klein. She openly calls him "the most handsome in Lord Roy's crew" and he actually calls her out when she snaps at Ellen for healing Klein before she gets a chance to herself.
• Fire Emblem: the Blazing Blade: Lady Priscilla of House Carleon and Raven aka Lord Raymond of House Cornwell. They even had a sort-of Childhood Marriage Promise going on. But later, this is subverted: Priscilla admits that she always was aware about him not being able to love her beyond siblinghood, and what she actually wanted was to stay by his side after many years of separation.

Magvel

• Fire Emblem: The Sacred Stones: Ephraim and Eirika (which is actually Twincest as well). In a nice reference, though, Ephraim's unique weapon is the spear Siegmund, while Eirika's is the sword Sieglinde, twins who fall in love from The Ring of the Nibelung. It even gets hilariously lampshaded in Fire Emblem: Heroes, where Ephraim tries to chalk this up to Malicious Slander but it STILL sounds like a Suspiciously Specific Denial (especially in the Japanese version, as the English adds the now memetic "disgusting").

Tellius

• Extending this to Not Blood Siblings, there's Sothe and Micaiah from Radiant Dawn. Despite referring to each other as like family many times, they're more or less the game's default couple to the point where they come in already A-supported. (Though the player can break this support if they so choose to.)
  • Heroes pushes the romantic angle even further with their Bridal Beloveds variant. Even though the default Micaiah explicitly refers to Sothe as her brother, the Bridal Duo has Sothe trailing off when he talks about Micaiah possibly marrying someone else, and also has Micaiah thinking "He Is All Grown Up" regarding Sothe himself.

Fates

• In Fire Emblem Fates, the Player Character can marry near everyone of the opposite gender... which includes his/her siblings from the Kingdom of Hoshido (Ryoma, Hinoka, Takumi and Sakura) AND her/his foster siblings from the Kingdom of Nohr (Xander, Camilla, Leo and Elise). Turns out BOTH sets of siblings are unrelated to said Player Character, however.
• One of the artbooks strongly, strongly implies that Leo's First Love was his half-sister Camilla.

Three Houses

• It's implied that one Noble House of the Adrestian Empire practiced this, "keeping their offspring pure" to keep a leash on their very valious Crest. It's House Nuvelle, founded by Apostle Noa, who additionally cast a spell to make the Crst of Noa look like the Crest of Macuil.

Heroes

• Book IV features twin siblings Freyr and Freyja, with the added plot point of the latter being a Yandere for the former. This is because when they were young, people mocked Freyja for the mark on the underside of her nose and Freyr was the only one who was kind to her and told her she was special. This led her to become obsessed with him, wanting to keep him by her side even if it meant tearing him away from the rest of the world.
  • Parodied at some point of said Book, when Alfonse is briefly hit with a spell that makes him a Chick Magnet - and his younger sister Sharena is among those affected by it. Hilarity Ensues.
• In book V, Prince Otr's devotion to his older brother Fafnir comes off as this. Subverted when it's revealed they're not really related, though Otr refers to him as "brother" until the end when Fafnir, now a possessed dragon, kills him.