One possible theme could be discipline. The older sister might be seen as taking on a parental role and using spanking as a way to enforce rules. However, this is a rather archaic view of discipline.
Another theme could be power dynamics. The older sister has the physical power to spank the younger one, which shows an imbalance. It could also explore themes of jealousy if the spanking is related to some form of perceived favoritism towards the little girl. But these are not positive themes and can be quite complex and uncomfortable.
There could be a theme of family hierarchy. In some traditional families, older siblings are given certain authority over younger ones. But in stories about spanking, it reflects a rather harsh side of that hierarchy. It may also touch on themes of forgiveness and reconciliation later on in the story, as the sisters might have to repair their relationship after such an incident, but the whole concept is still not something to be idealized as it involves an inappropriate form of discipline.
Chapter 141, ", Crumbles Upon Touch", is the clearest early answer because it turns Crumbles Upon's situation into a concrete plot event. The chapter gives readers a real answer by opening on The iron pipe rifles in the hands of those adventurers had no... and ending on At the same time, their loudspeakers were constantly relaying the....
Chapter 214, ", Mom's Plan", is the clearest early answer because it turns Tax Currency's situation into a concrete plot event. The chapter gives readers a real answer by opening on This thing, Gu Hang simply placed it on the rooftop terrace of the... and ending on By next year, I'm confident I can double that number." "Do you grasp....
Chapter 147, ", Offensive Initiative (6k subscriptions please!)", is the clearest early answer because it turns Reed Meshako's situation into a concrete plot event. The chapter gives readers a real answer by opening on waaagh!!" Roars of battle echoed from the numerous orc youngsters... and ending on This wasn't an issue of tactics or military skills but a matter of....
Chapter 177, ", Draining the Tech Card Prize Pool", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The chapter turns that into something concrete by moving from ...Four days ago, my unit completed the established combat plan and... to How could a hastily formed coalition withstand such a blow?. What makes it stick is that the chapter closes with As enemies, there were no pleasantries;
Chapter 221, ", if it were Yelisia, the quality of the offspring...", is the clearest early answer because it turns Gu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The Disciplinary Council was busy arresting people and conducting... and follows through with Wang Qi seemed a bit bewildered..
Chapter 210, "Sweeping Across the Eastern Provinces", is the clearest early answer because it turns Eastern Provinces's situation into a concrete plot event. It opens with For instance, they began to gradually accept the "advisory" influence... and closes on Latterly, Yan Fangxu simply made a summary-style military report....
Chapter 174, ", Loyal Heir Academy", is the clearest early answer because it turns Haier Si Morgan's situation into a concrete plot event. Early on, Gu Hang simply picked a few random classes and went in to listen to.... By the end, Only his most progressive wife managed to maintain high spirits, and....
Chapter 131, ", Battle of the Three Ponds", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter turns that into something concrete by moving from After receiving the telegram about Morgan and others escaping, Perbov... to But since they ultimately chose to rebel, they must have had....
Chapter 182, ", The Technical Expert You Wanted", is the clearest early answer because it turns Wu Jiarong's situation into a concrete plot event. Early on, Gu Hang could not let Wu Jiarong abandon her work.. By the end, A bowl of thin gruel to last a day.. Inside the broader power growth storyline, the chapter closes with It wasn't difficult;
Chapter 89, ", Ground Assault", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Gu Hang suddenly felt a palpitation. and ending on Although they only had a total of six hundred men, which seemed.... In story terms, the chapter closes with But just as she thought this, something unexpected happened!, which gives the main cast a stronger narrative position and pushes the power growth onward.