No. It is not a proper English expression. The word 'cum' is inappropriate in this context and makes the whole phrase seem very strange and not at all a normal way of communicating in English.
Once upon a time, there was a little boy. He loved his mom more than anything. Every day when his mom came home from work, he would run to her and give her a big hug. He would also pick little flowers from the garden and present them to his mom. He said those flowers were as beautiful as her.
For the son - cum - story, you could start with his birth, his first steps, and his first words. Then move on to his school days, his favorite subjects, and his extracurricular activities. As for the mom - story, talk about her pregnancy, her hopes and fears at that time, and how she prepared for the baby's arrival.
This could be about a mother teaching her son appropriate behavior in the context of storytelling. For example, she might be guiding him not to include inappropriate or vulgar content in the stories he tells.
This statement is really unclear and rather odd. 'Cum' might be a misspelling or an inappropriate term in this context. It could potentially be 'come' as in 'come stories' which still doesn't make much sense. Maybe it's a very local or made - up expression that is not commonly known.
Using that expression is not proper. Here is a story of a loving mother. A mother always cooked her child's favorite meals. When the child had to move away for college, she sent care packages full of home - cooked goodies regularly. She would also write long letters with words of encouragement and love, which made the child feel that her love was always there no matter how far away.
In a civilized society, we uphold values such as respect, family harmony and propriety. 'Cum in step mom stories' clearly violate these values and thus have no place in literature.