My story has. . . similar elements, I guess?
It doesn't involve a sister (not the time-stop storyline anyway) and it has hardcore sex, not only softcore nudity. The plot is that the boy leaves home, strips a girl outside, goes to school, strips more girls.
I don't think it's what you're thinking of, but you may find it an interesting read if you're into time-stop.
https://chyoa.com/chapter/Go-to-scho...Stop%5D.261728