Comment by malevolentslime

My 2 cents is that it's an easy way to describe/search the age and genders you want, without getting erroneous results.

So 2 younger people = bro/sis

An older man/younger woman would be Dad/daughter

A much older man/younger son would be granpda/grandson etc

It's hard to search "older man younger woman" because the adjectives will populate "younger man older women" too and skew search results.