Answer:
Whoever told you that Christian is my brother, was mistaken.
Step-by-step explanation:
"Whoever", is the relative pronoun.
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 predicate nominative.
A predicate nominative is a noun that completes a linking verb and renames the subject. A predicate nominative only exists after a linking verb.