As a key installment in the Game of Thrones franchise, it's not really a surprise that House of the Dragon has a notable level of incest. For various reasons, it's one of the fantasy saga's core tenets. As well as creating intentionally complex lines of succession that spawn their fair share of problems in-universe, the space where romance and platonic love meet opens some pretty compelling character dynamics.
All that being said, the ubiquity of incest in the Game of Thrones universe doesn't mean it can't still find chances to shock and confuse the audience.
House of the Dragon's season 3 premiere, "Salt and Sea, Fire and Blood," is a massive turning point in the story as both sides of the war for the Iron Throne clash at the Battle of the Gullet. Away from the action, the episode has several more intimate, character-driven moments that don't rely on visual spectacle. The unexpected kiss between Aemond Targaryen (Ewan Mitchell) and Alicent Hightower (Olivia Cooke) is one such scene, and it's drawn attention for being far more than just a quick peck on the cheek between mother and son. Mitchell has explained the moment from Aemond's perspective. It doesn't make things any less creepy, but it does provide some great context.

What Ewan Mitchell says about Aemond and Alicent's kiss
Speaking with Entertainment Weekly about the oddly standout moment in an episode filled with explosions and bloodshed, Mitchell added some much-needed context. "Aemond's trying to read between the lines," he said, "[to] see if there is an ulterior motive there." While the actor admitted to thinking the moment was quite "out there" when he first read the script, he went on to say that he "kind of saw it coming with everything that I've explored with Aemond and his relationship with Alicent."
That isn't where Mitchell's analysis ends, though. He also thinks that Aemond was "assuming control of the family" when his character planted a wet one on his mother's mouth. He also, fairly understandably, raised the question of how the kiss would impact the dynamic between both characters as House of the Dragon progresses. Furthermore, Mitchell made the astute observation that Aemond has never really been on the receiving end of unconditional love from his mother or anyone else.
"He really doesn't know how to express affection," Mitchell said. So, maybe this is just how all that comes out. Plus, I do think Aemond has some underlying feelings of attraction toward Alicent, and it's only now that he sits on the Iron Throne that he feels he can express them without consequences or the fear of being shunned. Still, it hits a little harder than the usual Targaryen-on-Targaryen action, given that Alicent has come from a family where incest isn't really a thing. The quiet horror in her eyes as it happens makes the moment pretty unbearable.

Aemon and Alicent's kiss may or may not happen in House of the Dragon's source material
George R.R. Martin never specifically writes in Fire & Blood that this moment happens. That being said, the book isn't a traditional novel, but rather a chronicle of Targaryen history in the centuries before the rebellion that sets Game of Thrones' core events in motion. As such, it would have been a little strange to randomly include a mention of Aemond and Alicent kissing in the book.
On the other hand, that doesn't rule out the possibility that it happened in the source material, just sort of "off-page." It's not directly relevant to Fire & Blood's main purpose, which is to fill in the historical blank spots as if it were written by Archmaester Gyldayn. With that in mind, I'd say it's very unlikely that Martin ever considered this scene, and that it's original to House of the Dragon.
Catch new episodes of House of the Dragon on HBO every Sunday night.
