I mistook you for your brother.
Watashi wa anata o otōto san to machigaemashita.
I; me; private affairs; personal matter; secrecy; selfishness
topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis
you (referring to someone of equal or lower status); dear (what a wife calls a husband)
indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.
younger brother; little brother; kid brother; pupil; apprentice
Mr.; Mrs.; Miss; Ms.; -san; makes words more polite (usu. in fixed expressions)
if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)
to make a mistake (in); to commit an error (e.g. in calculation); to confuse; to mistake something for something else