Definitely not. Sharing something like that with a sister is highly unethical. Family relationships are based on respect, love, and appropriate boundaries. This kind of sharing violates those boundaries and can cause discomfort, harm to the relationship, and is not acceptable in a civilized family environment.
This is extremely inappropriate behavior. Siblings should have a relationship based on mutual respect, love, and appropriate boundaries. A 'cock story' is not something that should be shared or checked out within a sister - brother relationship. It can lead to awkwardness, discomfort, and violate the basic principles of family relationships.
It is far from normal. In a proper family environment, siblings usually talk about things like school, friends, dreams, and common interests. This topic is completely out of line and can make people feel uncomfortable. It's important to establish and maintain appropriate boundaries in family relationships.
Such a story is not something that should be discussed as it is likely to be immoral and against social norms. It's important to focus on positive and appropriate content.