Comment by Xi Jinping on I’ll Retire After Saving the World - Chapter 94

@Xi Jinping It's meant to take place in the original timeline shortly after the mini disaster that happens in the early chapters where a quarter of the students don't make it because of something like a dungeon break. It's just showing the girl's reaction to the initial incident and what made her into the harsh older sister the main character grew up with. At least that's what I think the author intended.