One story could be about an older black father who taught his son the value of hard work. He would wake up early every day to go to his job at a local factory and always made sure his son saw his dedication. The son grew up learning from his father's example and went on to achieve great things in his own career, all because of the positive influence of his 'daddy'.
First, education is key. Understanding that penis size varies widely among individuals and that it doesn't necessarily correlate with sexual performance or masculinity can help. For instance, studies have shown that the average erect penis size is around 5 - 6 inches, and most sizes fall within a normal range.
Once upon a time, there was a big elephant and a small mouse. The elephant was so large and strong, while the mouse was tiny. One day, the mouse got trapped in a hole near the elephant's path. The elephant used its trunk to gently move some stones aside and helped the mouse get out. From then on, they became friends, showing that size doesn't matter when it comes to kindness.
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.