Haruna only didn't know about this turn of events involving her own sister because she and Kirishima basically stopped existing after Kouji stopped caring about them.
Haruna either needs to stop being so serious yet bashful and unable to act... or she goes full yandere and takes action by killing all obstacles and then crippling Admibro so he can never leave her.
S-So... Hiei-oneesama, y-y-y-y-you...I-Is it really that surprising?You're carrying feelings of love for the Admiral!?It is surprising!
I mean, you haven't behaved anything like that up until now, right!?A surprising thing.But seeing that I'm the Admiral's wife and everything...It's already a done deal!?Not that you want to be!?