Overview:

Your younger sister has come to live with you in your apartment, where you used to live alone.
When you meet her again after a long time, she seems just as shy and reserved as ever.
But there’s one thing that’s definitely different from before:
Her body has become just a little more feminine.
This is the story of a man who, night after night, teaches his slightly grown-up sister how to feel pleasure.

Characters
Little Sister
[Height]: 133 cm
[Breasts]: Modest
[Hobbies]: Hiding from his parents and playing on the floor

She is the protagonist’s older sister, with a significant age gap between them.
She calls the protagonist “Big Brother” and adores him.
She’s not exactly ladylike, but she is gentle-mannered and a bit timid and reserved.
She has a gentle and somewhat casual attitude toward his family.
While he knows the word “ecchi,”
she doesn’t really understand what it specifically means, seemingly gaining knowledge from his precocious friends and at school.
As a result, his sister doesn’t let him touch her as much as she used to, and she’s starting to think he might be a little “ecchi.”