I often mistake him his twin brother.

英訳に間違いが有れば正してください。

よく彼と双子との兄弟と間違えてしまう。
I often mistake him his twin brother.

ヒント
mistakeの使い方

答え
I often mistake him for his twin brother.とする。

mistake+目的語+for…で目的語とfor以下を間違える