The phrase is very ambiguous. 'Cum' might be a misspelling or an abbreviation that is not common. If'mom let me come in her stories' is what is meant, it could imply that the mother allowed the speaker to be included or be a character in her stories. But again, this is just speculation without further details.
One way to interpret it could be that'mom let me come in her stories' means the mother allowed the person to be incorporated into the stories she was creating. For example, if the mother was writing or telling fictional stories, she might have let the speaker be a character in those stories.
This is not an appropriate or ethical situation in the context of a traditional relationship. In a committed marriage, having an extramarital relationship with a 'boyfriend' goes against the vows and values of marriage.
The most obvious consequence could be the breakdown of the marriage. The husband, if he finds out, is likely to be extremely hurt and angry, leading to separation or divorce.
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.