"They follow their brother."
Translation:Loro seguono il loro fratello.
i thought one did not use il before family pronouns unless the noun is plural. or is it also for plural pronouns like loro?
"il loro" and "la loro" are the exception to the rule that the article is not used before singular family pronouns. (mio padre, tuo fratello, sua figlia, nostra madre, vostra sorella but il loro figlio or la loro madre etc).
I made a typing mistake (too fast) and omitted "il" before "loro," and was surprised that it was accepted. Is this a Duo error, or has the usage of "loro" changed in modern Italian?
I am not aware of any development in modern Italian regarding the use of "loro". Dropping the article before "loro" would sound unidiomatic. So, yes: it is a Duo error. I agree with jesslc (except that "madre", "padre", "cugino" in Italian, as in English, are nouns, not pronouns).