Hiyori's throwing a fit and it's definitely immature, but I think it's quite a good snapshot of her relationship with her sister and exactly why Akane and everyone else worries about her the way they do.
Hiyori's super dependent on Akane. Akane is probably the only person in the world that she would pitch a fit in front of, because Akane is an existence that's supposed to pamper her. The same way you might behave like a regular functioning adult most of the time, but then regress into a childish pile of goo when you visit your parents.
This has been and has always been the main conflict of the story. The isekai savior...got summoned with what's basically her mom. Hiyori hasn't been able to completely grow up because Akane's been there for her to fall back on. I think having the issue be this up and front means it's almost time for them to resolve it and for Hiyori to let go of her sister and become more independent, and not just discuss the possibility of it like they did a while back.