Incest stories for all its reasons will always remain fantasy stories, because in normal grounded setting having a close family sexual relationship is next to impossible. Therefore almost all writers have difficulty in developing incest relationships in "grounded" environment. It's forbidden & called taboo for the same reason. In most of the stories writers tend to ignore so many other factors which can effect the relationship of main characters as it makes it easier to remove the roadblocks cultural & societal norms can bring.
Part of the spice of taboo stories, is because they always have a level of forbidden nature that doesn't go away. If it's something you don't have to hide or something that progresses beyond a certain point, it might as well just be a normal relationship at that point.
logically a story can take several avenues to progress.
1) Hidden relationship. There's a stress in the air when they are in public, wanting to be closer, but at home are eagerly fulfilling their base desires. Heh, the song '
Don't Stand so close to me' encapsulating the feeling.
2) Secret boyfriend. This implied and i see more where siblings or family are lovers, and live a normal life, but when they don't get their rocks off at a date or are teased or the like, they can take their frustrations out.
3) Short term fling. This seems more likely the natural progression of any real incest relationship, where barriers are broke down for a while, maybe a summer, maybe a weekend, both parties recognize what was there and choose... to walk away, restoring the previous relationship but with a little more understanding of eachother from sharing a part of themselves. It would probably be a few years before they forget it even happened.
These all have limited durations, days, weeks, til they are of age to leave the house...
*shrugs* who knows. A lover, boyfriend/girlfriend that never truly leaves, who you have an innate connection with that never leaves...
4) Domination/Cuckold. While usually mother/son it could go other ways. But ultimately mom (or dad) is led into the arms of family, usually with the parent as a VERY willing party, an inner slut or who would have done it anyways if given a chance.
Regardless, if it's passing of the torch, replacing the bedroom's male/female figure, or just a fun fling behind hubby's back...
This might actually be more likely with Workaholics who are great husbands but don't give their wives enough attention.
5) Harem... Several girls or guys agreeing to share. In a world where we push monogomy this doesn't make sense, but in a polygamous worldview this would make a lot more sense.
I would think the seed/idea would have to be planted very young, like a bunch of kids playing and saying 'i'm going to have X as my husband' and them doing rock paper scissors or playing house and at some deep level they already feel like they are part of a group, so when they are older they fall back into the group. That or some kind of group pact when they find out as a group they are all interested, and set guidelines.
Then the emotional implications. Love, lust, societal expectations... I've roleplayed many of these scenarios, and emotionally they can be very enlightening, but highly unrealistic. Which is too bad. I'd have loved to sleep with my sister. But it would have probably been purely sexual... Or it would have crippled me unable to move forward.