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.
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.
One of the main negative impacts is the distortion of the child's view of sexuality. It can make the child prematurely exposed to adult - only concepts, leading to confusion. For example, he may not understand the appropriate boundaries in relationships.
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.
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.
Educate yourself about appropriate family values and boundaries. By understanding what is respectful and moral, you can easily recognize and avoid such inappropriate stories. Also, encourage those around you to uphold positive values.