My Sister’s Best Friend? My Female Friend Already? Then, What’s Next―?

Is there a “next step” in male/female friendship―?
Touma Itohara is not interested in love. He is comfortable with relationships where lines are drawn.
He’s already satisfied to be in a relationship that draws a line, and there’s no doubt that it’s fulfilling for him.
“―Onii-san, if you haven’t left yet, let’s go home together.”
But recently, there was a little change that occurred in his daily life. Rin Mamiya. She is his sister’s best friend.
A girl who often comes to their house… At first, their relationship was just that.
But now that they go to school together, they go home together, and hang out together on holidays as a matter of course.
They also also started to hang out on weekends, and their relationship grew stronger and stronger. But
Even so, Touma only treats her as a “friend of the opposite s*x.”
But on the other hand, Rin seems to have her own feelings about this relationship…?
This is a coming-of-age story and romantic comedy story.