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.
Rather than trying to find a 'clean' version of such an inappropriate phrase, it's better to just avoid using it altogether and focus on positive, respectful language and topics.
Since I don't have prior knowledge of 'Black Cum S3D stories', it might be some sort of experimental or independent art form that combines elements in a way that is not mainstream. Maybe it has something to do with a unique form of 3D storytelling with a specific color or thematic element related to 'black', but this is just speculation.
Yes, in some farming communities, there are stories about how black roosters are great at protecting the hen house from predators. Their loud crowing and brave nature are often part of these tales.