I'm not entirely sure as I haven't read this specific light novel. But generally, 'siscon' refers to sister complex and 'brocon' refers to brother complex. So it might be about the relationships between siblings with such complex feelings in the story.
The light novel might have a distinct art style if it has illustrations. This art style could enhance the portrayal of the complex relationships that are central to the siscon and brocon themes. Also, the story could be set in a unique fictional world that adds to the charm of the narrative.
The title 'ore no imouto ga saikou no okazudatta novel' seems to suggest that the story in the novel revolves around a person's sister. The 'saikou no okazudatta' part is a bit more ambiguous. It could imply that the sister has gotten into the best or worst kind of situation related to an obligation or debt. Maybe it's a story of how the protagonist deals with his sister's situation within the plot of the novel.
I'm not entirely sure as I haven't read this specific light novel. But generally, a light novel often has a unique story. It might be about a relationship with a sister - like character perhaps, given the 'imouto' (sister) in the title.
One of the main features could be the character development of the sister. Starting as a gal - like character, she may grow and change throughout the story. There could be plotlines about her making friends, dealing with school bullies or social situations due to her appearance and style. And of course, the relationship with her brother or other family members would be a central part of the story, with their interactions evolving over time.
Since I don't have detailed knowledge of this light novel, it could potentially be about the relationship between characters where the presence of a younger sister (imouto) is crucial. Maybe it tells a story of how she impacts the life of the protagonist, whether it's in a family - related, emotional or adventure - type of context.
I don't know for sure as I haven't read it. But probably there is a main character who has a strong connection with his or her sister, given the title.