In Volume 8 of the manga, Minami calling Miyuki "Onee-chan" (even though it was an accident).
Miyuki: So I suggested that we do something on the making and history of Paulownia chests of drawers, but... I'm ashamed to say...
Minami: (thinking about how Yutaka calls both Konata and Narumi "Onee-chan")
Miyuki: (notices that Minami's mind is somewhere else) Is something the matter?
Ah, no... It's nothing. Onee-chan... AH
Minami: AH! No, I mean... I'm sorry!
Miyuki: (remembers Konata and Yutaka addressing each other as "Yu-chan" and "Onee-chan") There's certainly nothing to apologize for, Mi-chan!
- After this, the next 4 panels has a conversation between Miyuki and Konata over the phone about the pressures of being an older sister and Miyuki silently complimenting Konata, Tsukasa, and Kagami:
Konata: Really? Minami-chan called you "onee-chan", Miyuki-san?
Miyuki: Yes. A long time ago, we were close... But this was the first time I ever heard her use such a family-like expression with me. It made me so happy... This was the first time I ever felt the pressure of being an onee-san, like you must feel everyday.
Konata: Miyuki-san, you're just amazing, so there's no need for you to feel pressure! Unlike me...
Miyuki: Thank you for the kind words but... I think the really amazing one is the friendship of Yutaka-san, who is causing changes in Minami every day.
Konata: Wow! For such praise from Miyuki-san, Yu-chan must be really amazing, huh?
Miyuki: Yes. I think I'm a little jealous.
Miyuki: (silently, to herself) That's something that could be said of you and the entire group, you know? But it's so embarrassing to say that the words won't come out of my mouth.