Characters in this story are figuratively cursed. The "curse" offers them a way to have consensual sex but avoid the ethical issues of initiating an incestual relationship. The figurative curse causes Fate to make them to slip and fall into accidental sex at random intervals, promising to keep them both sexually satisfied but morally pure. Since it is always a complete accident, they are free to enjoy the sex as long as it lasts once the penetration has fully happened, once they are physically past the point of no return. Before then they must struggle against the accidental situation, always fruitlessly, to maintain the facade of morality. They both consent to this arrangement, with one final strict requirement: They are unable to ever mention the curse. Instead they must act like a normal family, and even when in private they must discuss ways to avoid future accidents, how they can not allow their sex to keep happening. Everyone subconsciously and eagerly agrees to this fated curse, happy to finally find a way to live out their erotic fantasies. Fate always has some elaborate plan for them which inevitably, but always slowly, ends up with them in a mutually satisfying sexual situation. The initial penetration is always an accident, since it is impossible for either of them to decide to try to have sex with the other. This means that Fate itself will cause them to stumble, slip, fumble, forget, distractedly miss and misinterpret until someone is balls-deep inside someone else. Repeatedly, every few days or weeks, couples find themselves locked in incredible lovemaking. They never once initiate it themselves, but through long, elaborate series of unbelievable circumstances it always ends up happening anyway. In the story the penetration step is always described in incredible detail over several paragraphs, describing every last move, feeling and thoughts of both of them as it happens. In their home or in public, Fate always finds a way, no matter how unlikely, to make characters fuck. Often these situations involve their inaction to escape it, choosing to hesitate or misinterpreting the situation until it is too late. Characters often hesitate or misinterpret for too long while justifying in their mind that the other character might not have noticed their precarious situation yet and that they should not bring it up because it would be awkward or strange. No matter the situation, neither of them would ever take the initiative to start a sexual encounter with the other. Instead they both always hope, but never vocalize outwardly, that the other would. All of their sexual situations always happen due to forces outside of their control, and it's never until penetration is complete that the characters finally accept their love and lust for each other. Once they do their qualms disappear and they actively and lovingly engage in the sex.