Well, it might be the unique relationship between the characters. Maybe the way the story unfolds the duties and bonds within the sister relationship is what makes it special.
Sisterly love is a very common theme. It shows how sisters care for and support each other. Another common theme is the idea of shared responsibility, like taking care of their family or dealing with family secrets together.
One similar light novel could be 'My Little Sister Can't Be This Cute'. It also focuses on the relationship between siblings in a somewhat comical and heart - warming way.
One popular 'Sisters Duty' light novel could be 'Sisters Duty: The Bond of Family'. It often focuses on the strong relationships between sisters and the various duties they have towards each other and their family. Another might be 'Sisters Duty: In the Face of Adversity' which could explore how sisters support one another during difficult times.
The plot of 'A Sisters Duty' manhwa centers around the lives of sisters as they navigate various situations. The main themes could involve love, sacrifice, and the importance of sisterhood. There's a lot of emotional depth and character development.
Another similar light novel could be 'Onii - sama ni Sa - shite yo!'. It has themes related to family and growing up together, much like 'My sister's duty light novel'.
Maybe it's about a sister's journey in fulfilling certain duties. It could involve family bonds, loyalty, and perhaps sacrifice. For example, she might have to take care of her younger siblings, protect the family's secrets, or face challenges that are related to her role as a sister in the family or in a particular situation. Without reading it, this is just speculation though.
The unique portrayal of sisterly duty might be what makes it special. It could explore the responsibilities and bonds within a sister - sibling relationship in a way that other novels don't.