Insisting on translating "Otouto" & "Onee" into "little brother" and "big sister" is killing the wordplay in the series.
The protagonist's name sounds like "Otouto".
The protagonist's ability name is playing off of that.
The sister's name sounds like "Nee".
The "Néal" Order is based around turning all the girls in it into "Né"-sans.
It might actually be an intentional bilingual pun, with "Nee" + "all", but that'd need the author to confirm or deny.
The problem being: localizing those terms into "little brother" and "big sister"/"older sister" completely erases those connections.
Please do consider just using the Japanese terms; they're not complicated to pick up, and they're fairly widely-known within the demographics that'd be reading on Mangadex.