Honorifics are always important imo it ticks me off so much when translators just do away with them bc it tells you so much about the relationship between characters. The worst is when like onii/onee-chan is translated to big bro/sis it feels so unnatural literally no one calls their siblings bro or sis in English regularly.
Agreed, in the first place, changing onii-chan to Big Bro is a room temp IQ localization. Like you said, none of us call our brothers and sisters big bro or big sis like that, we call each other by name or nickname, so if you're gonna localize it for the west, you may as well just put their name there?
Or, just leave it how it is...if we wanted to read western slop, we'd be reading comics....we're reading manga because we want Japanese stuff, which includes their culture and how their language is structured. It's kinda gatekeepy, but if the new manga fans can't be bothered to learn the nuances, that's on them, maybe they shouldn't read manga.