YOU ARE DOING A TERRibly good job with the translation and I fully understand the tonal paradox you had to use when doing Haruyo's onee-sama register. I'd say it's a warranted paradox as even older English novels written by fair ladies also used similar registers, but I love how you accidentally made the two languages directly oppose each other in tone in order to retain the subtext of closeness.
This is so awesome, I can't wait for the next chapter! The last series that made me this invested Heart of Thorns (or something close to that I can't remember at the moment)