Little Hirata Tetsuya (later Miki Tetsuya), the second oldest boy in the family, is stuck babysitting his youngest sister, Suzu.
Children were expected to help out around the house, especially with their younger siblings. While carrying the younger sibling around was a task that usually fell to an older sister, the only other girl in their family was only 5 years old, making her too young to do so.
Tetsuya and Suzu were real people, two of Hirata Atsutane's grandchildren.