I'm not sure if you know this about your father, but he really wants to have another son. The older he gets, the more he wants to preserve his legacy. We've tried getting pregnant but it's not going well. You know what happens when men get older... they have trouble in the... erection department.
I know you want your father to be happy, don't you? This is really hard for me to ask, but will you help me get pregnant? Obviously your father doesn't know I'm talking to you. I know you find me attractive. Maybe you could take the love for your father & the sexual energy you feel for me & find it in your heart to do us this favor. If you just take your pants off, I'll do the rest. I know this is weird, but I have to give you a blowjob to get you hard. And it's okay if you enjoy it. I hear that if both parents are happy at conception, the resulting progeny will be happier. Just relax & let stepmother ride your cock. What if I turn around & ride you the other way? I bet you'll like that; your father sure does. As much as I love him, it's been a while since I've had a young cock.
Do you think you can come inside me? Do it for your daddy. Oh my! I didn't expect to orgasm but I guess I enjoyed it more that I thought I would. What?! You're ready to go again. Well that would increase my chances of getting pregnant. We'll probably have to do this again tomorrow too. You okay with that son? You do want to please your father don't you? And if we both enjoy it in the process, well that's better for everyone