Final answer:
To transform the sentence into a complex sentence with an adjective clause, add the adjective clause 'who is sitting next to you' to modify the noun 'boy'.
Step-by-step explanation:
To transform the simple sentence into a complex sentence with an adjective clause, we need to add an adjective clause to the sentence. An adjective clause provides more information about a noun or pronoun in the main clause. In this case, we can use the adjective clause 'who is sitting next to you' to modify the noun 'boy'. The transformed sentence would be: The boy who is sitting next to you is my brother.
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