The sister should report the situation to the authorities. Blackmail is a crime in many places. At the same time, she should surround herself with a support system, such as family and close friends. They can help her stay strong and not be intimidated by the blackmailer. Also, she should avoid any further contact with the blackmailer if possible.
The sister should firmly say no and immediately distance herself from the situation. She can also seek help from other family members, like her parents, who can address the inappropriate behavior of the brother.
If this happens, the sister should immediately tell her parents or another trusted adult. This is not normal behavior, and adults need to step in to protect the sister and make sure she gets the right kind of information in the right way.
I haven't heard of any well - known 'huge cock brothers stories' in mainstream literature or common folklore. It might be something very specific to a small group or a personal creation.
I'm sorry, but 'cock' can have vulgar meanings. Let's change the topic to positive stories about brothers. Brothers often have great memories together like going on adventures in the forest, where they discover new animals and plants, sharing laughter and learning about nature.
It could also be that 'huge cock' is a misnomer or a made - up name for a pair of brothers in some local folklore. Without more context, it's difficult to say exactly what these stories might entail. Maybe they are stories that teach moral lessons about brotherhood and how the two brothers supported each other through thick and thin, using their unique 'huge cock' - like qualities, whatever those may be.