This is a fundamental problem with "will they/won't they" as a source of appeal. It works for a movie or a single book, or as a romantic subplot in a story that has nothing to do with romance. but in longer narratives focused on romance, love or sex, there's a shelf life for how long two people who want to hook up can believably not hook up.
Not only narratives focused on romance/love/sex. How many series have this duet of characters in love but not in love but in fact yes they are in love but well they'll never tell the other that they love him/her ? And most of them quickly become boring just because of that. By fear to kill the interest if those characters finally end together, the writers slowly kill the series itself.
That's why I talked about the pace that become faster in the middle of the story. There's a progressive curve here, because the more you accept your love for the other, the faster you want to reach the next step. This until the moment where you finally give in, and have sex any time it's possible, like it happen at the start of any relationship.
And this apply even more for incest stories. The curve is more marked, because you don't just need to accept that you are in love with the other, you've also to deal with the taboo part. But during this time your love grow further and stronger ; you don't consider giving up to incest until you're sure that it's really what you want, and therefore that you really love the other. What mean that when something will finally happen, it will not just be a blow-job like it happen too often in those stories. With incest more than with any other relationship, it's all or nothing.
After the hookup, the formula has to change. Either to a narrative centered around the later stages of the new relationship, or else something has to happen that prevents the relationship from becoming the new status quo.
It's not necessarily a status quo, all depend of the context.
A long time divorced woman will want to catch up on lost time. Same for an incestuous relationship ; all this time lost because of a social convention that finally you decided to not follow. A young girl have everything to discover regarding sex, and there's still a lot that a less young one still don't know ; what is probably the reason why corruption games have such success, they can still progress once the characters start having sex.
Which just leaves the option every developer usually ends up taking:
Quickly wrap up the story and start a new one about some other, unrelated schmuck who has the hots for a family member.
Why not do it the hard way? By actually writing out all the juicy details of a long-term relationship as anne O'nymous suggested?
I think that it's mostly because the authors discover that an incest story stop to be an incest story the moment the characters start to have sex together. Of course, they are still related, but to their eyes, they are lovers before being, by example, mother and son. The taboo is just something that spice up their relationship.
MissFortune named
Now & Then, I think it's a really good example. The relationship was incestuous only during the few firsts updates, because it was the adoptive daughter. But quickly it became a standard relationship, two persons in love who happen to be related. Time to time there's a revival when a new character enter the scene, but it's only to the eyes of this new character that the relationship is incestuous, at no time the MC and his daughter see it that way, and even less act like this.
It's possible to make this last longer, by putting others characters living on the house. They then act as constant reminders that "this is not right". But there's a limit to how long it can be used.
One can also play the conscience crisis card. They are kissing in a public park, and the mother suddenly believe that a co-worker saw them ; "Oh my god, what am I doing ? He's my son, I shouldn't have had sex with him". But this card can only played once, and either it last one/two days, or it lead to a definitive break-up. You don't accept to have incestuous sex just because. You thought about this for a long time and when you're doing it, it's because you really want it.
Or maybe because the people who write these things have seen plenty of examples of forbidden love and whirlwind hookups in fiction, and relatively few examples of stable, entertaining long-term relationships.
I guess that most people believe that stable long-term incestuous relationship don't exist. I know they exist, because I knew person living one, but honestly even with this knowledge I would probably not be able to write it. What it really mean to be in love and related ? Is it more frustrating or more exciting ?
I mean, imagine a family diner. If you're there with your girlfriend or wife, you can take her hand, you can kiss her and all those small sign of affection. But all this is prohibited if your girlfriend is also your sister. So, do you pass the whole day frustrated to be so near, but also so far, to the one you love ? Or at the opposite, is it more exciting when you're finally alone together, and once again you've the night of your life ?
One of the incestuous couple I knew was in a kind of "friends with benefit" relationship. They were together unless one of them was in a relationship, and their own relationship would last until one of them find the love of his/her life, if it happen. Lost contact with them a long time ago, the fate to be 50yo, but last time I saw them, they were still happy with this.
Perhaps is it the solution for a good incest story that can last on (in game) time: not putting the incest at the center of the story.
But it would be then more on the side of a "slice of life", with months passing between two periods where they are together. Unless the MC can corrupt his girlfriend, making her accept that any family diners end with a threesome in the mother's bed. They would have their own relationship and life, and time to time incestuous sex would come to spice up things.
The conclusion being introduced when the said girlfriend say something like, "you know, I think that I'm starting to fall in love with your mother. I still love you, but I think that I also love her". They would then start talking ; is it really love, is it serious, do they all feel the same, and all. And after some times, they invent a pretext for the rest of the family, and buy a house where the three of them will live, having sex anytime they want, and not just after family diners.
Or, maybe you'll discover that it's a lot harder than you thought it was.
It's definitively harder, reason why one should plan his story first. He would then discover than most of them don't works because there's no way to end them well. But when he'll finally found the right one, he'll know exactly where he's going.