To create an objective summary of "The Crow-Child," the essential details to include would be:
- A brief description of the two main characters - This provides basic context about who the story is about, specifically Ruky Lynn and his sister Cora.
- The genre to which the story belongs - Knowing the genre can help frame the expectations for the story's themes and style.
The other options (analysis of the sibling relationship, lesson learned by the main character, and opinions on mistakes) would be more subjective and not strictly essential for an objective summary.