THere's something about strong assertive girls calling their sister Onee-chan rather than Onee-sama that is that much more delicious. Endearing > formal.
Satsu... ki... Onee... chan...!Hmph!Sa-... Sa...Kiryuuin Satsuki, why yooouuuu!Guh...! Nuh...!
HEY! THE HELL WITH THAT NOSEBLEEEEED!?You still have a long way to go!...~S-!What's the matter?! Hurry up and do it, Matoi Ryuuko!!