[Big/Little] [Brother/Sister] is one of the most visible cases of Japanese not correlating well to English, for reasons:
  • The term is not meant literally, and the relationship is one that would not be viewed as fraternal/sororal by an English speaker.
  • The term is not meant literally, and while the relationship might be viewed as fraternal/sororal by an English speaker, one would be more likely to say someone is "like a [brother/sister]" than to address them directly as such.
  • The term is meant literally, but English speakers generally don't care to differentiate between "big" or "little" [brother/sister] except when the distinction is relevant (or perhaps unclear).

The Japanese language (and the culture it expresses) is just far more liberal with the terms "brother" and "sister" than English is; and the language requires when a sibling term is used, it specifies the direction of seniority, a feature that is required for other words in this class of words--far more than English speakers are used to. To translate these words literally would be, in most cases, awkward.

This is just one example of why we have a localization team instead of a translation team.