I'm not aware of any specific "black cock prison white woman stories" in a legitimate context. Prisons should be places where the law is upheld fairly regardless of race or gender, and most stories should focus on the proper functioning of the prison system, inmates' rehabilitation, etc.
I'm not sure specifically what a 'big black prison cock story' is as it could be something very specific to a certain context, group or even an inappropriate or made - up thing. Without more information, it's hard to say.
Given that this term is so unorthodox and not in line with normal prison discourse, it's likely that it has no origin in the real prison - related research or knowledge. It could be a product of someone's inappropriate imagination or an attempt to spread false and offensive ideas. Prisons are about justice, safety, and the management of inmates, not about such inappropriate fictional stories.
I've not come across such a story. It could be a regional or personal story that hasn't spread widely. Maybe it was made up by someone in a prison setting or by those who have an interest in prison - related fictional tales, but that's just a guess.
Such stories often involve very disturbing and unethical content. However, in a civilized and legal society, prisons are supposed to be places of rehabilitation and protection of rights. While there may be individual untrue rumors or fictional accounts, the vast majority of prisons have strict regulations to prevent any form of sexual abuse.
There are many more positive and educational topics related to prisons, like the work of prison educators or the efforts to reduce recidivism, which are much more worthy of discussion.
There are many great works that deal with prisons in a profound way, such as Aleksandr Solzhenitsyn's works which expose the harshness of the Soviet prison system, but again, not in the context of something inappropriate. We should look for the deeper meaning and positive messages in prison - related literature.
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.