mistook, confused, got confused
Desculpe; parece que eu o confundi.
Sorry; it seems I confused you.
Não tive a intenção de confundi-lo.
I didn't mean to confuse him.
Eu o confundi com o irmão dele.
I mistook him for his brother.
A princípio eu o confundi com seu irmão.
At first I mistook him for his brother.
Eu a confundi com a irmã dela.
I mistook her for her sister.