"I am the driver's niece."
Translation:Eu sou sobrinha do motorista.
It seems to me that occupations that end with the suffix corresponding to -ist in English end with the suffix -ista in Portuguese (and maybe in Italian too—I am thinking of barista). In addition the masculine and the feminine are the same, only the articles, pronouns and adjectives change for the genre.
Actually, it must be a romance language thing. It's the same in my native French with the suffix -iste, for instance un grand artiste/une grande artiste
It's unclear if "the" refers to the driver or to the niece. Normally, you would think there is one specific driver and people don't necessarily know whether there is a niece at all. So in that case, I don't understand why "eu sou uma sobrinha do motorista" would be wrong (was marked wrong by Duolingo). I wrote "uma sobrinha" because normally you could say "I am a niece of the driver". Let me know if I'm getting this all wrong haha :)