。
She is often mistaken for her sister.
Literal
[topic-は] often her sister [with-と] is-mistaken.
間違えられる is the passive of 間違える ('to mistake X for Y'). The standard of comparison Y is marked with と ('as'), which here functions as the identity-comparison marker: 妹と間違えられる ('be mistaken [as identical] with her sister'). This と differs from comitative と ('with [a partner]') — it points to the entity one is being equated with. Note that the sentence redundantly says 彼女の妹 ('her sister') — a quirk of literal English-to-Japanese translation that a native writer would normally omit, since 彼女は already establishes the topic.