For me, there is a lot of significance on why Hinata addresses Neji as “older brother”. Figuring that out, completely demolishes any attempt to argue that Hinata is comfortable with the slave system in the Hyugas.
And it isn’t just to get “closer to him” or because she’s just so “kind and warm”. Well, she is kind and warm and gentle and loves her brother and her sister. But that honorific alone crosses the bounds of it.
In every culture the elders hold importance and respect, but this especially goes in East Asian culture, particularly rooted in Confucianism. In Japan, it is also known as senpai-kohai (senior-junior). As its name suggests, it describes hierarchical structures, the younger must respect the elder and the elder must guide the younger. This works mainly in institutions and social groups (-san, -sama, -chan, -kun etc) , but this moral code more or less, has naturally reflected into homes and families as well. But even without mentioning senpai-kohai, we’re all pretty much informed about how in Japan, there are honorifics used for elders, how “you should listen to your elders.” Again, this isn’t just exclusive to Japan or Asia, so not only we are knowledgeable in it, we also relate to it to some degree.
We know hierarchy is an already established tradition within the Hyugas. Hiashi became the head just because he was minutes older than Hizashi, not to mention the main and the branch family themselves is based on, maybe a little more complicated than age differences, but still is based on one.
What this all means, is that Hinata isn’t just referring Neji as an actual family member rather than a servant. Hinata is giving him a degree of authority over her to him. She, a member of the main family, is putting him,a member of the branch family, above her in some way, hence the honorific she uses. Hinata, my insane girl, does not give a shit 👆😍🫶
We don’t have any other proof whether younger main members refer to older branch members with honorifics. Another testimony is Hanabi who also refers Neji as older brother, however it’s not a strong antithesis as firstly, this was done in a filler episode (390, Naruto shippuden) Secondly, even if the filler episode holds validity, Hanabi is then again, Hinatas younger sister which has most likely influenced her to address Neji the same way. Additionally, main branch members lead the lower with fear (the caged bird curse), in heavy contrast with Hinatas anti confrontational nature. I doubt there’s anyone else who does the same thing as Hinata and possibly Hanabi.
I don’t think this is a bare minimum in any way, I think for a girl who has never had a choice , is a hell of a way to express disapproval with what is happening and it’s really a shame Kishimoto just dumped it aside or off screened the development of Neji, Hinata, Hanabi and the Hyugas in general.
PS: My view and my interpretation of senpai kohai isn’t entirely correct but rather on what I’ve read and observed. This is based over my opinion and I would love to hear yours!!!