They always do different stories for each volume. Contrary to the OVA, I believe the story mostly revolves around Norio and kunimasa and the merman who is targetting norio (I think). Each story is meant to elaborate on the strangeness that is the madararui and then make them relatable to us as readers (as well as play with other animal species). I think Yonekuni isn't a big focus because he's honestly pretty normal contrary to how the author originally made Kunimasa out to be the normal brother, however, he's been twisted because of his grooming to be the next head of the madarame. (which I think is also the head of all the madararui)
tl;dr big bro and shiro will probably appear at the end of the volumes or as extras because I'm pretty sure they're just gonna be seen starting their family. Maybe they'll do a wedding special for them or something? Its all gonna be in context with Kunimasa because they're pretty much twins.
i bought the remaining chaps and i was so confused because it is a diff couple... like what about the others? where's the rest of their stories? i wanna see big bro and shiro ;V