Answer:
<u>Whoever </u>told you that <u>Christian is my brother</u>, was mistaken.
Explanation:
"Whoever", is the <em>relative pronoun</em>.
A relative pronoun is a type of pronoun that often introduces dependent (or relative) clauses in sentences. They also can stand alone as the subject or object of a sentence. There is a specific list of relative pronouns, and here they are: who, whoever, whom, whomever, that, which, when, where, and whose.
"Christian is my brother", is the<em> predicate nominative</em>.
A predicate nominative is a noun that completes a linking verb and renames the subject. A predicate nominative only exists after a linking verb.