。
She mistook me for my older brother.
Literal
She [topic-は] me [object-を] older-brother [as-と] mistook.
The frame Xを Yと間違える ('mistake X for Y') puts the actually-present person in the を slot and the misidentified target in the と slot. Here the misidentification involves 兄 ('older brother'), the counterpart to 弟 ('younger brother') — Japanese kinship terms force the speaker to commit to relative age, leaving no neutral 'brother' that ducks the question. Mistaking siblings for one another is a classic family-resemblance scenario, especially when years apart meant the speaker is now closer in appearance to a sibling than they once were.