Seirei Gensouki Light Novel: Inconsistent Honorifics

[Zerio]

I'm reading this novel since it's on catchup. I noticed that the honorifics in this series are all over the place.

Latifa called Rio Onii-chan. Miharu used Aki-chan, Masato-kun, Satsuki-san, and Takahisa-kun.
But Aki and Masato just used Miharu (in the anime, they called her Miharu-oneechan).
And the Saga family way of switching between Rio-dono and Rio-sama is ignored.

These differences felt weird. So, I'm curious if there's a certain logic on why the honorifics are inconsistent.

Especially vol 4.
Why is Miharu different from Aki and Masato?
Why does Latifa called Rio Onii-chan, while Masato doesn't call Aki-neechan?

Why not stick with one or the other?
Maybe if I understand the reasoning, my reading experience will feel less jarring.