So in your game, both of your characters engaging in incest have to have a subconscious emotional need that goes beyond wanting to fuck their mom or son. Their desire to fulfill that subconscious need will cause them to act irrationally, starting that incestuous relationship. But having them fuck because of pure sexual desire for one another will come off extremely unbelievable; instead it must happen as a byproduct of both of their desires for something else.
Exactly this.
Keep is just simple. To use cliche to express this :
If the son saw his mother topless while she's changing, the dialog goes like this :
- son "Oops, sorry mom."
- mother "No worry, can you wait outside, please ?"
- son "Of course."
And that all. No, "Omg he saw me naked", and other, "Why did it make me wet ?", coming from the mother. And no "Wow what amazing boobs", and other, "I shouldn't have a boner when looking at my mom's boobs", coming from the son.
For both of them it's natural, not because they are mother and son, but because of their inner desire.
Then slowly make it escalate. Another time, in the same situation, once the son start to go out of the room :
- mother "Wait, can you give me a hand to put my bra on."
- The son go back and help her.
- mother "Thanks you, I always struggle with this one."
- son "Why did you bough it in this case ?"
- mother "Well, I like how I look in it. Don't my breasts look firmer in it ?"
- son "I agree."
And once again, no, "I like to feel his stare on my boobs", coming from the mother, and others, "Your boobs always looks great mom", coming from the son.
Step by step, unprovoked situation after unprovoked situation, they slowly give in their inner desire. Each one of those situation felt so natural, that they unconsciously accepted a little bit of this desire.
Then it start to be more touchy, but still not in a sexual way.
It can start by a feet massage, absolutely not sensual in is intent, but felt like it by both. But still, without the cliché dialog lines. No need for a, "It make me so wet".
It's not the mother mind that react to the sensuality, but her body. She'll spread her legs a little, involuntarily, unconsciously she want to "expose" herself to the person she's in love to. And the son will obviously no say the classical "OMG I can see mom's panties", he say nothing, he thought nothing, he just please the person he love by reducing her pain with a massage.
Then the mother will ask him to also massage her ankle, because a massage is always something relaxing, and you always discover how tense a part or your body is, when the part right beside it isn't tense anymore. And it will go further and further, on the same massage, until the mother ask him to stop.
She was hit by her desire, suddenly realizing that she want more from him that just a massage. But what she feel is more confusion than shame. She don't stopped him because she's ashamed of her desire, but confused by the fact that this desire exist.
But all this without moaning, at least not in a sexual way, nor pervert thoughts and introspection regarding this desire.
Starting this moment, the mother's desire for her son isn't anymore an inner desire, she's fully aware of it. She'll probably struggle with it for some times. She'll be a little more tense when asking her son to go outside of the room, not ask him again to help her with her bra...
This until the moment she understand that it's stupid. She really struggle with this fucking bra, there's no reason to not ask for help just because she love her son more than a mother is supposed to.
And now this not anymore inner desire start to slowly grow into a conscious and voluntarily assumed desire. But still an unexpressed one. What she feel is love, not lust, therefore she'll not enforce it on her son. Like any lovers, what would hurt her isn't to not have sex with her son, but to see him unhappy. Therefore, in her mind it's clear, if they cross the line one day, it should come from him.
Make the son pass through the same journey. One day his inner desire will not be inner anymore, and at first he will avoid his mother, like she did with him. Here again until the moment he realize that it's stupid.
And for him also, in his mind it's clear, if they cross the line one day, it should come from her.
Then come the moment of extreme sexual tension, but still fully natural.
One day they'll kiss, a kiss intended to be on the check but who, for whatever reason, end to be on the lips. Don't know, it can be a goodbye kiss on a crowed place, and one of them is hit in the back. But it's still just a kiss, and once again, no "OMG" and things like that.
Then one day the mother kiss her son directly on the lips and not on the check (once again still just a kiss). There's nothing sexual in this kiss, it's just a way she found to innocently steal a little pleasure from the one she love. If the son say something (which isn't necessary) she can just say that it's faster like this.
Note that it must come from the mother the first few times, simply because she's the mother. When he was a baby, she's the one that kissed him on the check for the first time. And now she's the one that decided that now the kiss will be on the lips. Then, after few times the son will adapt, being happy to innocently steal a little pleasure from the one he love.
And it continue like this, slowly increasing but will still being totally not sexual. The mother's bra can be smaller than she thought when she bough it (it's a myth to think that size is something universal), and she ask for a breast massage. But nothing sexual, no moaning while he do this, instead she'll more surely say how much she's stupid to not have tried it before buying it, and how clothes designer can agree once for all to effectively have a 95 C bra that have the same size than all the other 95 C bras.
They can be in hurry, and share a shower. But avoid the usual boner of the son. He'll be half hard, because of his feeling for his mother, but that's all. He's not a pervert full of lust, he can't control his desire and the reaction of his body, but he still fully acknowledge that this shower isn't at all a sexual situation.
Then one day, it will be stronger than them. They'll be back home after a lovely day full of mutual happiness. They'll stand in the living-room, and the mother will say something like "I really enjoyed this day, but I'm a little tired, I'll directly go to bed." And she'll kiss goodnight her son, on the lips as usual now.
But this time something have changed...
The day was so lovely, she don't want it to stop, she want it to be even more lovely, and slowly, almost unconsciously, her lips start to spread apart. And like the son feel exactly the same, his lips respond... It's a matter of two/three second before they start french kissing each others.
Their lips spreading apart, responding to each others, what the sign they were waiting for all this time. Whatever who's lips started to move, it was an insignificant move. What is significant is that the other responded to it, showing that (s)he wanted it as much.
Another two/three seconds, and the french kiss is a furious one. The mother isn't tired anymore, now that she know that her love and desire are shared, she can't be stopped. It's a dam that exploded, they can't contain the desire they feel since so many time, the desire that was growing all this time.
There's no "I'll just give you a handjob", no "A blowjob but it's all you'll get". They'll have sex directly, they'll consume their love the instant they discover that it's a shared one.
And there's no shame, no "sorry talk" after this. What they did isn't something wrong. They are two persons that share a strong love feeling for each others, being lovers is the natural consequence of this.
But there's a "should we continue" talk, a none judgmental one of course. It should come from the mother, since as mother she's the caring one.
"You're still young, I'm 20 years older than you, one day you'll want to start a family on your own, to have children, I can't give you any of this". It's all, those are the only worries she have in mind. She's concerned by her son future, not by morale or what other would think.
And the son will answer with something like, "I don't know." ; "know", not "care", he's at the start of a love story, so confused about the future, he isn't a selfish lustful asshole. "For now the only thing I know is that I'm happy laying to your side and that I don't want to go out of bed. Last night was marvelous. Because of the sex, obviously, but more than this, because I was near to the person I love, sharing this love with her."
And that's all, the talk will end here. Everything have been told. They are related, but they aren't different to any other lovers. They know what they feel now, they know that, right now, they want to feel it forever. But like any other lovers, they don't know the future and don't know how their feeling will evolve with time, while not caring about this. It's the first day of their love story, they'll have all the time they need to think about what the future can be... and like any lovers, they'll never think about what the future can be.