"Did you follow your brother?"
Translation:Hai seguito tuo fratello?
It was wrong because there is a rule that says there is no definitive article (il, la) before possessive adjective (tuo) if we are referring to a singular family member. this rule also extends to "marito è moglie" (husband and wife).
But note that in plural there is an article:
Hai seguito tuo fratello ? / Hai seguito i tuoi fratelli ?
"Seguisti vostro fratello". Ok, so I know they were looking for the compound past in the answer, but I was curious if they would accept this answer. They didn't.
Italian speakers: isn't this technically correct? "Seguisti" is the 'passato remoto', which is pretty rare now, I know, but wouldn't this literally mean "did you follow your (plural) brother?"