Teasing the Sister-in-Law

"...It all began that day. The day my brother brought her home..."
When the protagonist's brother introduces him to his fiancée Miki, he is struck hard the moment he lays eyes on her.
Because she is the very woman he once secretly harbored feelings for.
They had lost contact without even knowing each other's names, and he thought they would never meet again...
While the protagonist is reeling in confusion, his brother and Miki get married.
Yet barely into their newlywed life, his brother leaves home for a long overseas business trip.
The protagonist begins living together with his parents and Miki, who has now become his sister-in-law.
And so begins his days of living under the same roof as his sister-in-law Miki...
As they spend their days together, the protagonist gradually can no longer hold back his feelings for her.
Until one night—
He happens to stumble upon her in the act of masturbating.
Witnessing this, his long-suppressed emotions erupt all at once!
He finally pushes her down onto the bed...!
The panicked, resisting Miki, however, gradually grows honest in body amid the pleasure she has not felt in so long, surrendering to his every move...
The protagonist takes Miki, pouring every ounce of his desire into her until the peak.
After the passion fades, the protagonist is overcome with deep regret.
Miki suggests they both forget this ever happened, saying "I'm partly to blame too," and the protagonist nods in agreement...
...However,
once the forbidden relationship is breached, it burns far more fiercely than either of them expected.
The protagonist, unable to stop craving Miki... Miki, resisting with her reason yet responding with her body...
The sister-in-law and her brother-in-law, plunging into an abyss of desire from which there is no return...
Where does this forbidden relationship ultimately lead...?