This is a regular recurring theme in the Fire Emblem series. Several of the girls in the player's party, well, admire their brothers a bit too much, with or without some bits of censorship. Examples:
Fire Emblem: Genealogy of the Holy War: Princess Raquesis and King Eldigan of Nordion (she even straightforwardly says she won't marry a guy who's less cool than her brother.)
Even the villagers seem to be aware of it. One village lady gossips about Eldigan and Raquesis being a little too close for siblings.
It's hinted that Eldigan still considers Raquesis his Most Important Person, though, even if he does love his wife Grahnye. Sigurd says in chapter 2 that he cares about his sister more than anything, and Raquesis' name is on his lips just before he dies.
The Oosawa manga makes this even more blatant, with them aknowledging in their last talk that they're in love ( with Raquesis almost asking Eldigan to be her first, but he refuses to 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 Raquesis to join Sigurd's trope in open hopes to get her killed in battle and is cold and indifferent to Eldigan's death unless it makes Raquesis 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.)
Cousins Ares (son of Eldigan and his NPC wife Granhye) and Nanna (daughter of Raquesis and Finn) have a special lovers dialogue which directly discusses the trope as it alluded to Eldigan and Raquesis' past.
What Could Have Been: As noted on that page, Raquesis was nefariously close to becoming Ares' mother during development...
Yet another example for Genealogy of the Holy War are the High Priest Claude and the Dancer Sylvia, whom many fans believe are really brother and sister. They both possess the Blagi bloodline, and Claude once mentions that Sylvia reminds him of his sister, who disappeared many years ago (and Sylvia 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). Though an official book shows that they're distant cousins.
Ayra and Holyn can get together. Both have minor Odo blood meaning they are at least distantly related. And in the Fujimori manga they double as Childhood Friend Romance, as Holyn always had a crush on Ayra but she forgot it as she grew up... save for her dreams. Once she recognized Holyn as the boy she literally dreamed of, they got together.
Ayra can have kids from Lex, and you can pair her daughter Larcei with either Johan or Johalva, nephews of Lex. Even more so: Larcei has a crush and can hook up with Shanan, who is her half-cousin regardless of who her father is.
If not paired with anyone else, Arthur and Tinny (children of Tiltyu) gets a special dialogue about her past and hinting that Arthur *may* have this kinda feeling to Tinny.
Actually in the case of Tiltyu's children, it is Tinny who gives the vibe of this one-sided feeling to her brother. Check her ending conversations:
Seliph : Tinny, youíll be going with Arthur too, arenít you?
Tinny : I, I want to be of some help to my brother, thatís all.
Seliph : You and Arthur are really close to each other, eh?
Tinny : Thatís because...I like him...
Seliph : Tinny, youíll be going with Arthur too, arenít you? Have you forgotten [character] already?
Tinny : Ummm, no, I donít think I ever would, but Iíll just try to do my best at going on with my life. Iíve met a very wonderful brother after all, weíll be trying our hardest together.*
* (The translator is TRYING HARD not to make it sound like incest)
There's also the fact that Julia, daughter of Arvis and Deirdre, has a crush on Seliph, son of Sigurd and... Deirdre, too. Must run in the family, dammit.
Seliph and Julia don't really count since they don't get anywhere and their feelings are left rather ambiguous (this is before they even find out they are siblings). Plus, the game data is specifically designed to reduce their Relationship Values over time. (If they're not directly adjacent)
Except there are two methods that allows you to hook them up. One of them involves the fact that love growths for being adjacent occur even if the characters are not of the same affiliation (Us, Them, Ally, Neutral). The natural love growth only occurs if both characters are of the same affiliation and neither is in the main castle. The other involves the hidden jealousy system and the fact that if you go from the first gen straight to the second, Julia has the higher priority for Seliph until you reset the game.
And oh, Seliph's other Love Interests seem to feel bad. Especially Lana for Julia should they engage in their "Lover Conversation" on the final chapter.
Seliph: "But I've always liked you, Lana..."
Lana: "I know... but it's just hard for me to accept that, Seliph. I feel so awful to Julia..."
What Could Have Been:...and just like the Eldigan/Raquesis example, originally, Seliph and Julia would've automatically become lovers during Chapter 8...
...and to add to that, in beta, Deirdre was originally going to be Vylon's daughter. Vylon is Sigurd's father. That meant that the Official Couplealso dodged the bullet in being this trope (in the actual game, Dierdre's father is Kurth), assuming Sigurd's father wasn't someone else at that stage.
Deirdre and Lord Arvis. Note that in the latter's case.... Having the two hook up and breed is actually a part of the Big Bad Manfroy'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.
Actually, according to the designer notes, Arvis 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 would change it at that point.
Special mention needs to go to the ability to pair up Lana and Lester with their cousins Faval and Patty. Their mothers are sisters, so from a social standpoint they're cousins. However, their mothers are actually Adean and Briggid... 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.
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 on it.
There's a theory going around that Lalum may be Oujay's long lost little sister. Their supports are filled with Lalum endlessly teasing Oujay.
Fire Emblem: Lady Priscilla of House Carleon and Raven aka Lord Raymond of House Cornwell. They even had a sort-of Childhood Marriage Promise going on... and she intended to ask him to fulfill it. Too bad that he does love her... just not in that way, but is more intrested in marrying his "nagging wife" Lucius... Then again, Priscilla can fall for others, though it doesn't always work. Amusingly, she and Clarine are both Troubadors.
Here's an interesting case: In Fire Emblem it's possible to hook up Hector with one of the Pegasus sisters, Farina or Florina, and pair Eliwood with the third sister, Fiora. Note that Eliwood's son Roy and Hector's daughter Lilina can marry each other in The Binding Blade. Hilarity Ensues.
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.
In fact, both Prince Innes (if you get his supports) and Prince Lyon are jealous of Ephraim because they don't think Eirika could be interested in another man. Of course, they both have their own issues (with the former sporting some kind of hatecrush on Ephraim and the latter... well, let's just say not in his right mind) so they are not exactly reliable barometers.
And Fire Emblem Awakening also indulges in incest, though mostly of the Kissing Cousins type. Chrom's daughter Lucina can marry Owain, son of her paternal aunt Lissa (though they're described as "companions" outside of Japan rather than "husband and wife"), and you can potentially build up other cousin pairings if you wish so (i.e., marry Owain to a Chrom-fathered Cynthia or Kjelle).
With the exception of the Male Avatar/Aversa pairing below (and even then, the latter is the adopted henchwoman of the former's father, Awakening staunchly averts Brother-Sister Incest for once. Chrom cannot marry his sisters, and he has healthy, sibling-like relationships with both of them! Also, if any of the future children have an opposite sex siblingnote Lucina can have either Brady, Inigo, or male Morgan as a brother, while female Morgan can be the younger sister of anyone else other than Lucina, they cannot marry either.
If you have a male!Robin father female!Morgan with Lucina and pair up Chrom with either Olivia or Maribelle... you can then pair up girl!Morgan with Chrom!Inigo and/or Chrom!Brady. Similarly, Lucina can marry a male!Morgan fathered by a girl!Robin with a Brady or Inigo fathered by Chrom. Meaning you potentially can get a niece/uncle or a nephew/aunt pairing via tweaking games mechanics. ... Uh, yeah, nothing to see here.
And then there are the bandit brothers Victor and Vincent, who are, in the game's own words, "eerily fond" of each other. This also gets played up in their voice acting, as they both like to call each other "daaaarling" for example...
And lastly, if you're looking for an outright brother/sister pair, there's male!Avatar with Aversa. note Spoilered due to Luke, I Am Your Father and the latter character being a Secret Character. However, they're Not Blood Siblings, as the latter was adopted. If you call "Validar murdering a little girl's family due to her powers, brainwashing her into believing she owed him her life AND making her super ultra devoted to him" 'adoption', that is