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 sure what specific 'bigger cock brother sister stories' you are referring to. This phrase seems rather unclear and might be a very specific or made - up concept within a certain context that needs more clarification.
No. Such content is often inappropriate and may involve violating ethical or moral boundaries. It's better to focus on positive, healthy and family - friendly topics.
One story could be about a brother who saves up his pocket money to buy his sister a special birthday present she's always wanted. It shows his love and thoughtfulness for her.
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.
There is not enough context to accurately define what 'brother big cock stories' are. It might be a very unique term within a particular community or family, perhaps some family - related anecdotes that are named in this rather strange way. But again, this is just speculation.