The term "Honorable Sister" is most probably a translation of the word "One-esan," which is a common term used by children to address their older sisters. Although the word does have an inherent element of respect associated with it, it is not quite as stilted an address as it might sound in direct translation, in English.
Yoko and Ko have a very normal sisterly relationship. Ko is looked up to by Yoko because she is older, and it is true that Ko is dependable, and a great help to her mother in many ways, including looking out for her little sister. Ko is not perfect, however, and, like any normal teenager, is subject to moodiness on occasion, and is sometimes frustrated by her little sister Yoko's behavior, especially considering the stress the family is under as they struggle to escape to safety during the War. Ko can be bossy and cross when she is annoyed. Yoko, on her part, looks up to her big sister, but clashes with her at times as well. Yoko is eleven years old, but as the youngest child she can be a little spoiled. Throughout the ordeal described in the course of the narrative, she grows up, and begins to look at things from a more mature perspective. The clashes between Yoko and Ko diminish, as Yoko is able to see things from Ko's point of view, and appreciate the difficulties that she is facing as well, as the older sister and one responsible.