When a Japanese says he can’t understand why English will differentiate between singular and plural but won’t differentiate between older and younger sister (like the Japanese language) then I could say the same thing about Japanese not differentiating between singular and plural. Circular argument.
A language must “decide” on what to emphasise due to language resource limits. It can’t do all work like some kind of Jack-of-all-trade. The interesting thing about language is which things in communication it decides to highlight. The singular is emphasised in English. Relative age is emphasised in Japanese. That reflects on the culture. So whether the culture or the language came first is as impossible to answer as the chicken or egg question.