because their relationship since they were reborn is clearly normal sibling banter
it's just ruby's word choice and example 'siblings kissing' is ambiguous, and can be misinterpreted by someone with an overactive imagination.
I'll conclude this with one of my favorite quotes in this manga:
Honestly Akane(that's her name right?) should know Aqua good enough. That that relationship isn't really real.
But her pissed off bitchy behavior won't change anything. She either has to accept he has a GF and calm tf down or just leave because what's she's pulling off there ist just mean
@usawion actually he can in my opinion, after all finding their father is only Aqua CURRENT goal. But, in the story right now we have another character who isn't only Aqua. We got Ruby with her dream becoming Idol, we got Arima who have a crush to Aqua, and etc. So the the characters still have much to tell.
And, even if Aqua already got his conclusion. He can find another goal. Maybe his bond with his sister and his surrounding can help him find it.
But, I think this mystery won't be resolve soon.
But... Idk, Aka always find a ways to subvert the expectation. Maybe, he will reveal it suddenly like a boom.