This statement is very strange and disturbing. It could be a very abnormal or made - up scenario. Maybe it was a misunderstanding or a part of a very strange dream that was misinterpreted as a real 'story'.
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.
Forgiving him depends on a lot of factors. If he is truly remorseful and willing to work on the relationship, it might be possible. But it won't be easy and will take time.
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.
One time, my little sister and I went on a hike in the mountains. We saw beautiful wildflowers and a cute little stream. It was a great bonding experience.