You have to consider that "brother" and "sister" are derived from Anglo-Saxon schools of etymology, likely hailing from the more Germanic branches of the English language as compared to its equally influential Romance connotations. While I'm well aware that these two words do have prominence as "black" colloquialisms, they are, in the end, WASP words at their core and WASP culture (and most of what comes from it) is implicitly regarded as the static reference point for most other cultures. Bronita, however, sounds deliberately Hispanic without a sliver of a doubt.
That's my take on it but I appreciate your view on the subject, lol.