彼女とお姉さんを間違えてしまった。

she; her; her; girlfriend; sweetheart

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

honorific/polite/humble prefix

elder sister; (vocative) young lady; miss (referring to a waitress, etc.); ma'am (used by geisha to refer to their superiors)

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.

to make a mistake (in); to commit an error (e.g. in calculation); to confuse; to mistake something for something else