One trope that works quite well is basically dependency, to generalize it. Be it that either people are all that each of them have (A Vision of God is a pretty good example of that), or where one person is at least implied to be sort of dependent on the other, but not necessarily literally so (Izzy and Me is a pretty good example of that). I tend to find that there's some amount of "realistic" emotional depth there, insofar as that's possible at least.
I usually try to devise triggers plausible to me and hopefully others.
For all its cliché, I think this situation is a good midway point in many cases, provided you can find justification that exceeds simple casual nudism as local normal.
Here is how it worked out in three 1-page episodes, one per POV. I hope the trigger is plausible enough.
I started a thread recently, about 'realistic' incest stories. When I say realistic, I mean something similiar to what you are saying. When you have a story with good motivation and development of the characters.
https://forum.literotica.com/showthread.php?t=1497510
I think there are a lot of plausible storylines, you just need to write them well
On the serious note, I'll try to exclude all of the mind-control, rape, blackmail and other similar types of scenarios. For me, a good incest story has genuine characters who have consensual sex because they developed attraction. Also, they need to live together, because 'separated on the birth' scenarios or 'coming back home after 15 years' scenarios somehow lose the taboo moment of the story.
Perfect example for me is https://www.literotica.com/s/mom-will-do-anything-ch-01 and https://www.literotica.com/s/mom-will-do-anything-ch-02 by Lovecraft68.
Plausible driving motivations:
- They kind of have to have sex, like in the Lovecraft68's story. They need money, but they are actually against this idea in the beggining. Slowly, they start to look at eachother in a different light and start being turned on by the other person.
- Outside factor. I had this girlfriend and we found out that we are both turned on by incest. Of course, neither of us wanted to do anything with our families. Then I met her mom who was smoking hot. My biggest turn on is m/d incest, so naturally I started developing my imagination. I was very open with this girl, so I felt free to talk to her about that. She never looked her mom that way before and nothing ever even came close to happening between two of them, but we had a bunch of really hot sessions fantasizing about her mom and she. In fiction, it is easy to imagine how the things could have developed further.
- Some incident where one person sees other in the sexual way for the first time. That's why I don't mind stories where moms and sisters look like pornstars. Let's be real, it is more plausible to be attracted to hot brother or cousin than to some who's not hot. If son has hot mom and he sees her masturbating or having sex with another woman, he might develop something. This is by far the most popular scenario in incest fiction, but majority of times it is written really poor. Like, they immediately start fucking after the incident. I think it is popular because it is most plausible scenario. That's why you need to be careful while writing it. Maybe don't start with someone seeing family member this way, maybe start with someone obsessed with incest porn, but disgusted by idea of actually doing it. Then, the thoughts start to creep in, his sister is dressing really provocatively, her brother is hitting the gym and all the girls are drooling after him... you get the idea...
- They are isolated for the longer period. All the stories on the cruises, in the mountain cabins, even when someone is injured (or in house arrest) and is confined to the home. So, they are alone, they don't have anyone to fuck, they don't even have some good material for masturbation, and what do they do?
- When the characters are in a really unusual surroundings. All the stories with mardi grass, summer vacations where they realize they are in swingers resort, I even found one where mom finds a job which organizes crazy sex parties and son accidentally, through his girlfriend, ends up there. It is similar to the first concept where 'they have to have sex' only here they don't really have to do that, they just follow the craziness of the place and situation.
- This is maybe too specific, but the scenarios which involve mom who starts feeling like she is too old and not attractive. Son or daughter (or both) start doing things to make her feel young and attractive again, but the things get out of the control.
I'm starting to be too specific and not writing about general dynamic, so I'll stop here. The point is, the key to all of these scenarios is good character development, with natural reactions of all of the participants.
