"My sisters are doctors."
Translation:Meine Schwestern sind Ärztinnen.
Absolutely correct. 'Aerztin' is an old word for a female doctor. In terms of equality, we call a female doctor 'Arzt', as well. Both answers should be accepted!
It said, "Meine Schwestern sind Ärztinnen," not "Meine Schwestern sind Ärzte."
This is a cultural question rather than a grammatical one. I know in Canada that we no longer use distinctions like waiter/waitress, or steward/stewardess and even the distinction between actor/actress is obsolete. In Germany do they still use gender specific language when speaking about occupations? Please note I am not asking about the vocabulary, I understand it.