Hiyori is a breath of fresh air for being this mature given she's only 14(?) and has better takes than her brother. She's even unfazed by the box of condoms or the idea her brother and his (potential girl)friend might be banging on the regular. She was also rightly worried when she learned they weren't dating. Cohabitation without the connection of a relationship is pretty abnormal, no matter how anyone spins it given they're not roommates at a share house.
Best of all she's not the brocon imouto stereotype so common nowadays in romcoms. It's tiring to see that archetype, it was funny when it first started showing up but now it's bad that it's practically a staple trope. At this rate she might get a bf (if she doesn't already have one) before Akira and Aoi start dating. Although she seems sensible enough not to do that before she's a little older. IIRC she's still a middle schooler after all.
A part of me is shouting that they should just give in and start dating already. But the cold hard reality has the specter of an LDR staring at them in the face. Something needs to be done about that if they actually resolve each other's emotional readiness part.