Sharing 'cock cum stories' with your sister is extremely inappropriate. Family is a place of love, care and positive interaction. Such vulgar content has no place in family communication. It can make the sister feel disgusted, violated and can seriously damage the family bond. Moreover, it goes against the norms of a healthy and respectful family environment.
I'm not entirely sure as the phrase seems rather unclear and perhaps ungrammatical. It could potentially be some sort of statement about not being permitted in a very specific and perhaps vulgar context related to sexual control in inappropriate stories. However, it's not a common or proper phrase.
It depends on creating a vivid and sensual setting. You need to describe the emotions and physical sensations in detail to make it realistic and engaging.
There are many great alternatives. For example, we could have stories about a sleeping boy's dreams of becoming a superhero, full of exciting adventures in his dream world.
We should not be looking for inappropriate stories. Consider the story of Pinocchio, a wooden boy who wants to become a real boy. His journey is filled with lessons about honesty and growing up. It is a classic story that has been loved for generations.