I think that sometimes, incest games are a stand-in for recreating sexual desires, experiences, and/or memories from an adolescent standpoint. I don't mean "loli" or "child" stuff, and I don't mean adolescent in an "immature" lame way--I mean literally adolescent sexual memories and experiences. Just good old 13-16 year old teenage exploration stuff. I like playing incest games and enjoy good incest erotica not because I harbor fantasies of my sister (don't have one) or my mom (that's honestly pretty gross to me, even if I don't mind in games/stories). In a good incest game/story, there are characters who either like sex but need a lot of convincing to take sexual steps because of the relationship (b/s, f/d, m/s etc), or are new to sex but are exploring their sexuality in a "safe" place with someone they love and trust because of the relationship. For me, these games plug into teenage desires and experiences. I never had the fortune (or misfortune?) of hooking up with a promiscuous girl as a teenager, but well before I was 18 I was with a few girls who had their "boundaries" they expected me to respect, but there was always an element of seduction (corruption?) at play to break down those boundaries.
Since it's, err, challenging, to create sex content depicting 14-16 year-old teenagers exploring sex together, moving past boundaries, seducing and/or corrupting until they're "going all the way" (which I think is a common teenage experience), incest games tap into those desires/memories in a more realistic way then depicting two 18-year olds who, for some reason, have placed boundaries on how far they'll explore. This might be even more true for people growing up in religious or otherwise "strict" households where sex itself was taboo, where any physical interaction with the opposite sex was taboo even if it was normal by most standards.
All that being said, of course there are plenty who enjoy incest games/stories because their first sexual "awakenings" and awareness involved a family member (my best friend in HS had an older sister who was hands down the hottest girl in the school, and although we teased him of course, I never asked him in any deeper way, but I can't even imagine that he lived with and around her growing up without one day realizing that prettiest face and best ass and most perfect tits in town were right there in his own house!).
TL;DR Incest creates a natural, inherent sexual boundary, a boundary many of us experienced in our awkward teenage years, which in turn creates the context for sexual conquest that mirrors those early experiences trying to push through that boundary.