I posted this comment to another thread, but it seems appropriate here, and I've made some edits.
I think that sometimes, incest games are a stand-in for recreating sexual desires, experiences, and/or memories from an adolescent standpoint. I definitely 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 personally like playing incest games and enjoy good incest erotica not because I harbor fantasies of my sister (I don't have a sister) 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 (this works particularly well with b/s and f/d plots). 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 "having fun" with a few girls who had their boundaries that they expected me to respect. I tried to respect those boundaries because otherwise I'd be cut off, right?, but there was always an element of seduction (corruption?) at play to break down those boundaries. Incest plots work the same way--go too fast or too pervy and game over. Take it slow, and step by step break through those boundaries, and you are rewarded. Those boundaries rarely exist when two people in their 20s or 30s are hooking up, so that's what I mean by recreating adolescent experiences.
Since it's, err, challenging, to create sex content depicting 14-16 year-old teenagers exploring sex together while moving past boundaries, seducing (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 are not related but, 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 "forbidden", where any physical interaction with the opposite sex was taboo even if it was normal by most standards. For those whose sexual growth was mitigated by feelings of guilt or shame (religious or otherwise), incest games place the MC in a familiar situation of approaching sex as a taboo subject, even if in-real-life incest was never in play as a motivation or fetish.
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. I have no personal experience with this, but my best friend in HS had an older sister who was hands down the hottest girl in our high school. We we teased him of course, but I wonder what it was like to live with her growing up and then one day realizing, at around 13-15 years old, that the 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.