"Dia punya seorang paman yang bekerja di sebuah bank."
Terjemahan:She has an uncle who works in a bank.
"That" should be correct.
"It is sometimes argued that, in relative clauses, that should be used for non-human references, while who should be used for human references: a house that overlooks the park but the woman who lives next door. In practice, while it is true to say that who is restricted to human references, the function of that is flexible. It has been used for human and non-human references since at least the 11th century, and is invaluable where both a person and a thing is being referred to, as in a person or thing that is believed to bring bad luck."
Normally "who" is for the persons, and "that" for the things, when "who" refers directly to a person. An uncle who.
If you say: His uncle is so mad that... you would use "that", because it introduces a consequence.
But as it's said in dictionaries, in the use, that is also correct, but in doubt, maybe use rather "who" when it's a person, you will be sure to make no error.
Karena kalimat ini bukan menunjukkan subjek punya seorang paman, tapi memberitahu bahwa pamannya bekerja di bank.
Sedangkan whose itu untuk kepemilikan.. contoh "A handphone whose i use is yours" sebuah hp yang aku gunakan adalah milikmu. Atau kalo kalimat pertanyaannya "whose car is it?"