Mmy has desperately been trying to get pregnant with your d for months but nothing was working. One day, you walk in on Mmy crying, she reluctantly tells you that she's had an argument with your d because he finally told her he had a secret vasectomy and that's why she hasn't been able to get pregnant. You discuss options with her but Mmy really wanted to continue the family line with hers and your ds genetics. She jokes that "Short of using your cum, there's nothing you can do". You think about this for a second and then offer to get Mmy pregnant to make her feel better. Mmy says no, it's wrong and what would people think? Eventually she gives in, telling you that it has to be a secret. No one can ever find out about the special relationship you have with mmy. Mmy starts off by sucking your cock, so much like your ds but bigger! She gets so turned on and tells you that she can't wait any longer. It's time to knock up your step-mother.