Steve Rogers' kiss with Sharon Carter may be weird after the events of Avengers: Endgame - but it's not incestuous. In the MCU, the great love of Captain America's life was his beloved Peggy Carter. Tragically, at the end of Captain America: The First Avenger, Steve Rogers sacrificed his chances of a happily-ever-after with Peggy, crashing the Red Skull's plane in the Greenland ice cap. He awoke 70 years later, with Peggy now an elderly woman in a nursing home.
It turns out that was only the beginning of the story, though. At the end of Avengers: Endgame, Captain America decided it was time to retire from the superhero life - and in the most beautiful and poignant way. He used Pym Particles to travel back in time, and returned to his life with Peggy, claiming the dance he had always dreamed of. According to writers Christopher Markus and Stephen McFeely, Cap arrived sometime around 1948, and since then he's been a background presence in the MCU. Captain America: The Winter Soldier established that Peggy had married and had two kids; as far as Markus and McFeely are concerned, the father of Peggy's children was always Steve Rogers. It neatly explains why the elderly Peggy only ever had photos of Steve at her bedside, and never of the supposedly different man she married.
As sweet and romantic as this may sound, one detail has rather grossed the fans out. In Captain America: Civil War, Steve shared a passionate kiss with Sharon Carter, Peggy's great-niece. It had always been rather strange and awkward, with Captain America clearly transposing his feelings for Peggy onto Sharon; the kiss even happened shortly after Peggy's funeral. But Avengers: Endgame seems to make it even worse. Technically, Captain America was kissing his own great-niece.
While Cap's fate certainly makes the scene more than a little uncomfortable, it's technically not incestuous. Sharon's surname is "Carter," which means she's from the Carter side of the family, not the Rogers one; she's the granddaughter of Peggy's brother or sister. She isn't a blood relation to Steve, and so this isn't incest. This was probably an intentional decision on Marvel's part, made when they realized how Cap's story was going to end; Captain America's return to the past was locked in from the very first draft of Avengers: Endgame, and Markus and McFeely had begun work on that script while Captain America: Civil War was still in production. In the comics, Sharon is actually Peggy's granddaughter, and that would have been a whole lot more problematic.
It could have been a lot worse, too; rather than explaining away a single tender kiss, there could have been a full-blown relationship. In an interview with Collider last year, Markus and McFeely revealed that their first drafts for Avengers: Infinity War had included the return of Sharon Carter, who they'd planned to be in an actual relationship with Steve, with the two even living together. Thankfully, that got dropped when they hit upon the idea of focusing the plot of Avengers: Infinity War on Thanos. "The movie does not have time to catch up on 'previously in the life of Steve Rogers,'" Markus observed.
Captain America's ultimate fate does add one more element of cringe to the kiss, though. Psychologists have long noticed that people tend to be drawn to members of the opposite sex who remind them of family members, a phenomenon known as "positive sexual imprinting." The stronger the relationship with the loved one, the more likely a person is to be drawn to someone with the same or similar characteristics. Presumably Sharon was pretty close to her uncle, and as a result she was attracted to a man with the same kind of character. She couldn't know that, because of time travel, she actually was kissing her uncle. A counselor would have a field day with this one.
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