Stepbrother and sister must watch pornography together but they get horny

He heard some strange noises coming out of his sister's room, sure she was watching one of her weird videos, curiosity got the better of him and he approached carefully, however, when he stuck his ear to the door he understood those noises. His little sister was masturbating and moaning shamelessly. He opened the door to disturb and was met with the image of her spread-eagled, touching herself while watching a porn video on her phone. He had a wicked idea, he would force her to do whatever he wanted to her or he would tell his parents everything. His sister agreed, she was wet enough to receive him in her vagina.