The primary reason that I’d put forward is that Japanese people place a lot of importance on social hierarchy, to the point that even between twins, it’s important to know which is the older sibling. Because it’s used in everyday conversation and in referring to one another (although not quite as much with twins, I just brought that up for emphasis).

The point is that the Japanese version of these words are used a lot more and have a lot of extra meaning compared to the English phrase “Big Brother”. So, it’s actually a worse fit. Japanese people use enough English loanwords that they probably understand the English phrase, anyways. So, the meaning gets through without the extra unintended baggage from the translation.

It’s one of the many pitfalls of translation. Often, there are words that mean “the same thing”, but they still aren’t the right words because either the extra connotations in the original language or the extra connotations in the target language can throw off the translation.

I think the Japanese translation is fortunate that, in this case, the Japanese language already has so many English loanwords… although I’m not sure whether that was exactly the same case when the book was first translated. It was published in 1948, I think. My recollection is that the English loanword boom started after WW2, so that would be somewhat contemporary.