1. let's talk register
2. Register is the varying degree of formality in language. Japanese has 3 primary registers: casual, polite, super polite.
3. Mostly this is expressed through the equivalent of verbal punctuation: desu/masu forms of words.
4. Kasumi pointing out Haruyo being overly polite with "masu", breaking "Onee-sama's" character.
5. AriSaku (aka Dear Flowers, Flowers of Yore, whatever the collective eventually settles into calling this manga) is crazy heavy on register changes.
6. Haruyo writes politely and elaborately as "Onee-sama," (which breaks down after she learns the truth), speaks very childishly at home, is naively respectful with Kasumi-senpai, and is familiarly casual with "Sumi-chan."
7. She may jump between them on the same page or even the same panel.
8. Kasumi sulking in part because Haruyo swapped from "Sumi-chan" to "Senpai."
9. "Su-mi..."
10. "Senpai, what do you need?"
11. Kasumi was always distantly respectful writing to Onee-sama, but after meeting Haruyo she switches to a casual demeanor, like speaking to a kid.
12. With their "S" relationship continuing, it's no spoiler to say there will be a lot of register switching coming up.
13. However this can be an ordeal to cleanly express in English (in skinny speech bubbles), so please bear with me and read between the lines.
14. If it helps, I brainstormed a little cheat-sheet while translating to establish character voices:
15. Haruyo. Kouhai: Deferential, immature, hesitant. Onee-sama: Clipped, assertive, refined.
16. Kasumi. Senpai: Casual, direct, omitting. Little Sister: Polite, reserved, passive.
17. Anyway that was all you needed to know.
18. If you really needed to know that?
19. The rest will be example dialogue you really don't need to know.
20. You can stay late to watch the teacher scribble on the board if you're a nerd.