This kind of interest is not in line with the norms of a healthy relationship. A good marriage relationship should be built on positive emotions, common goals and positive communication. You can try to introduce some positive literary works or movies to your wife to guide her interests to a more positive direction.
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.
No, it's not appropriate. In most cultures and for reasons of privacy and healthy boundaries, parents and children should have separate bathing times as children grow older.
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.
No, it's not normal. Gloryhole is a concept that is often related to sexual or inappropriate situations. If your mom has an interest in stories about it, it could potentially be a sign of her being exposed to inappropriate content or having a rather strange fascination. You may want to gently guide her towards more positive and appropriate forms of entertainment or storytelling.