I put "the nun is bringing an orange" and it was incorrect, though the translation says that schwester means sister, nun or nurse...
You are right. My first thought was of a nurse. But while in colloquial language it is all three, more exactly a nurse is a Krankenschwester and a nun is a Klosterschwester (oder Nonne).
I put "brought." What would have the sentence looked like in past tense using brought?
Die Schwester brachte eine Orange. (Präteritum = past tense)
Die Schwester hat eine Orange gebracht (Perfekt = present perfect)
Unlike English, German generally uses the past tense and the present perfect more or less interchangeably, so both are valid translations of the English past tense. Which one you choose is not so much a question of grammar, but of style, with the present perfect being more colloquial. Note that the verb "bringen", like the English verb "to bring", is irregular, i.e. you say "I brought" (not "I bringed") and "Ich brachte" (not "ich bringte").
Is this like a relative-sister, or nun-sister? Cause if the former, that's being pretty mean to your relatives.