Script: The story would be that it's your step-son's 18th birthday, and you start reminiscing with the about the day he was born. You get a little carried away and mention that you actually had an orgasm while giving birth to him. You notice that actually turns him on, and you want him to get into his "birthday suit", since you haven't since him naked in a while. You also get naked, and encourage him to start jerking off. You strip down as well to show him the body he came out of 18 years ago. The rest of the dirty talk I would leave up to you (although mention stuff like how breastfeeding him used to turn you on, etc)