Hitozuma ga Wakai Juuyoku ni Otosareru Toki

In the ordinary salaryman household of the Machida family, life was uneventful.
Husband Tsuyoshi, son Satoshi, and wife Haruka lived their days in quiet routine.
That routine was disrupted when Tsuyoshi’s nephew, Masato, came to live with them.
After losing both parents in an accident, Masato was taken in by his closest relative, Tsuyoshi.
Haruka and Satoshi had barely met Masato before. Now, Masato—the same age as Satoshi—transferred to Satoshi’s school. Handsome, academically gifted, and mature beyond his years, Masato quickly overshadowed the quiet, unremarkable Satoshi, leaving him feeling inadequate.
With Tsuyoshi often away due to work, Masato stepped in to support Haruka. Unbeknownst to Satoshi, Tsuyoshi began relying more and more on Masato, and Haruka, too, found herself depending on him.
Then came a turning point.
One night, when Tsuyoshi was away on a business trip and Satoshi was staying out for a club activity, Masato confessed his feelings to Haruka.
To his aunt, he admitted that he had come to see her not as family, but as a woman.
Haruka was shocked. But as Masato revealed his vulnerabilities—something she had never seen in him before—she found herself unable to resist.
What should have been a one-time mistake, known only to the two of them, was witnessed by someone else. Haruka had no idea.
That secret would change her—as a wife, as a mother, and as a woman.

[Translated from dlsite]