Fire Emblem: it's got incest. I don't know about you, but I'd heard about that one long before playing a single game.
I'm not going to wonder why this is so prominently featured, to the point where FE4 was allegedly
toned down prior to release yet still contains it as a plot point. But, let's explore it a bit, and not in the kink meme way.
Humans appear to have a
mechanism, or
mechanisms, to avoid boinking their siblings. Have you ever seen a romantic couple where they looked, and even maybe acted, like siblings and yet weren't? They're pretty common. In fact, there appears to be a phenomenon known as
genetic sexual attraction where people do indeed click in every possible way with close blood relatives-- provided they've been strangers for the early part of their lives. The
Westermarck effect is thought to have developed to defuse that attraction; basically, if kids are raised together, they end up not being interested in one another in that way. Even if they're not blood relations. There's a critical period for this reverse imprinting, though-- the kids need to get desensitized to one another before the age of six or seven.
That actually meshes pretty well with some of the FE Incest (TM) that we see. Alvis and Dierdre met as adults. Yuria didn't encounter Celice until they were teenagers. In the Oosawa manga, where the Eltoshan/Lachesis relationship is played to the very hilt of an Earth Sword, they also meet in adolescence. And genetic sexual attraction is off and running...
Some obvious counterexamples come to mind straight off, though. If you interpret Priscilla's interest in Raven/Raymond as incestuous, it's a bit harder to explain because they were together as kids. Ephraim and Eirika have been together since conception, and the game script points to their forever-togetherness as rooted in twinship, Westermarck effect be damned.
And then there's the incest fandom doesn't talk about nearly as much. The relationships between siblings not, as the SKU phrases it, "bound by blood." Well, you say, then it's not incest. Oh yeah?
1) Leaf and Nanna. So cute. So sweet. So... wait, how many times in the Thracia script does he call her his sister before they decide to make couplehood official? They've been raised together since infancy. The sexual-attraction thing should not be happening here. Even if you throw evo psych out the window, it surprises me that I've never seen one person saying that Leaf/Nanna squicks them. Yeah, we know they're not related. They know they're not related. But, stiil... the script pounds down repeatedly on the siblinghood business, right up to and including his proposal of marriage.
2) Altenna and Arione. Even weirder than the above, because Altenna does think he's her biological brother. Arione doesn't, or at least not by the time of the Big Reveal (how can he, with Trabant kvetching about Ethlin's ghost while Arione stands right there?), but they're still into one another. Totally, completely, canonical OTP into one another. And the Big Reveal just gives them permission to get it on without breaking any taboos.
3) Sothe and Micaiah. Which does squick me out, a little. Their relationship seems to be a little more complicated than a straight out sibling dynamic, though, and I think that's where the squick lies.
(Cellica and Alm were not, as far as I can tell, raised in the same household, and so I do not think they fall into this category. And there is no, repeat NO evidence that Marth and Caeda knew one another as very young children. All the kiddie refugees in Tirnanogue potentially fall into this category, though.)
On a final note, the Westermarck effect is supposedly depressed in younger siblings who are quite a bit (eight or more years) younger than their elder siblings. FE case in point, Maria and Michalis. But the elder sibling is NOT supposed to reciprocate the attraction to a sibling they remember as a squalling newborn.
More examples and counterexamples are welcome.