You mean “sibling zone”. They're definitely not just friends.
With all that said…
There's nothing I like more than sweet vanilla romance. That of course includes spousal fidelity, so I'd normally be inclined to ship Hinako with Yuusuke.
Y'know what else is included in sweet vanilla romance? Not neglecting your spouse.
So if Hinako turns out to receive zero affection/intimacy from Yuusuke, I'll start shipping her with Mitsuba with “FIGHT ME BRO” levels of conviction. It wouldn't be a pleasant thing, but it would be a thing.
Always glad to see a fellow shipper in the forums here, and I agree, that while that does count as infidelity, which I hate with every fiber in my body, I cannot spare any remorse for husbands/wives who neglect the spouse on purpose, or stop caring about them after sometime into the marriage (which I believe is a huge complaint in Japan, reasoning why cheating is kinda overlooked by society as long as it stays unknown). It's one thing not being able to be with your partner, either due to work or any other good reason, and both couples communicate it, and make sure it remains in the short term. But on purpose, nah.
Though I think in this case, we're not seeing a lot of Yuusuke since the focus is on her and Mitsuba's cute relationship. Personally don't see it going the shota x oneesan route.