Actually, Inazuma knows. Zui was just saying she's not sharing her onee-san.
Edit: Ah, whoops, I didn't notice her looking off in the 2nd panel.
She's looking to her left while Akatsuki's on her right so it's still rather ambiguous, actually.
But since she intentionally went with "Don't say nothing more!", I guess she knew.
Eh?Having an onee-chan like Shoukaku-san, I mean.Riiight?I'm jealous of you, Zuikaku-chan.I want a big sis like that too.Hawawah!?Nooo!!Ahahaha, whyyy's that?poooutTrue, she's a kind and wonderful person, the sort who wouldn't get angry with you.Don't say nothing more!