俺の妹は今日からドＳ ～お、お兄ちゃんなんか、あたしのペットにしてやるんだからっ～

The sister's name is Aina, and the brother's name is Haruki.
They're a little closer than the average brother and sister, and they have a relationship where they regularly do things like normal blowjobs.
But Haruki isn't completely satisfied.
He's a masochist who gets excited when women scold or insult him.

One day, he was secretly watching an adult video that was right up his alley.
"I want Aina to bully me like this and make me cum..."
he thought as he masturbated, and before he knew it, Aina was next to him...