in this story at least, Akatsuki and Sazanami position is as the onee-san, so even though Ushio is older than Akatsuki, they will call her onee-san. Heck even Oboro calling Ikazuchi okaa-san this doujin has some weird things happening at how they address people
wait Ushio chan is younger than Akatsuki? Because she called her Onee san, and they're smaller than Ushio chan....care to explain please?
Ushio is actually Akatsuki's immediate older sister (Special Type DD #20 to Special Type DD #21). Ushio is, however, being considerate here, due to Akatsuki's reaction earlier. Note that both Akatsuki and "nee" are bolded here, indicating that Ushio is deliberately being careful with her word choice there.
Think of it as Hibiki's onee-san, with (Hibiki's) being implied. Adults will sometimes say it to (usually other people's) children as well 「お姉さんはえらいなぁ!」 "You're such a good older sister!"
You're wrong! This is... I just thought... thought... that since I couldn't finish this, that I'd give half...
Hyaaa!I won't be able to finish this...Say what...!?It's so good...Oh, Hibiki-chan...I-Is that so! That's great!I wonder where Oboro's got to?Shall I have a bit, then?If anyone saw this, then there'd be a misunderstanding...How this began.WAAAH!
Akatsuki-oneesan!If you head over towards the mess, you'll probably meet Ikazuchi.My! How lovey-dovey.Hibiki, youkan really isn't something that you go at like an ice-cream, now is it?She's oblivious!?Since that girl likes to move around by herself...All that's left is Ikazuchi and Oboro, yeah!