I 100% agree Robin's content is incest, genetics are second fiddle to the actual familial relationships imo.
For whatever it is worth, Robin's scenes aren't written with much of the incest fetish in mind. Her whole point was that she didn't want to risk her daughter-father relationship with Simon, but she also needed to air out the fact she had sexual feelings for him, knowing that succubi had the "best of both worlds" and since Simon was now an Incubus King with a huge harem, she felt confident enough to at least talk it out with him. But the scenes themselves are mostly just written with a lot of intimacy, not really making a kink out of the exact nature of their relationship or mentioning it all that much.
Also, I think the game does good enough setup that you can't really hold it against Simon, he had blocked sexual feelings for Robin as you'd expect him to do in a normal parental relationship, but the situation in the game became one where Robin had already matured into an independent adult woman, she had sexual feelings for him, and his sex life situation had already turned into something extremely far removed from "normal". They talked it out and then over the course of several months Simon worked into unblocking his sexual feelings for her as blocking them was something that no longer served their relationship, they managed to do that while still keeping their familial affection of a father and a daughter that was no longer dependant on him and things went on from there.
The content might be "a bridge too far" for some people and I completely understand that (dev has expressed she kind of wishes she had made Robin optional as to not alienate players that cannot agree with their relationship halfway through the game, but that would be a big branch on the game's content and she really wanted to have Robin and Simon's relationship in it), but thematically I think it fits the game and it was definitely not shoehorned in, deviating from the beaten path and being willing to do what actually works best for different people and different situations is a general theme of the game, and so is the fact that the different and often unusual sexual relationships Simon has with his harem are not an obstacle to the real underlying affection between the people involved as long as all involved parties are okay with it and work it out.