When a brother, albeit a stepbrother, sleeps with his sister in the same room, sex between them will happen sooner or later. The smell of her body, her rounded forms will make any guy masturbate. And the pictures of her in his smartphone turned his sister on. She felt like an adult magazine star. And she wanted to thank her brother for the experience. In her own womanly way... And it was satisfying for both of them.
It was a strange gangbang between these two couples. For some reason, the brunette was more satisfied, even though the blonde was prettier. I would have switched to her.