The phrase Julian is nonrestrictive, or rather, nonessential, because of the meaning of the sentence. The sentence says youngest brother - this means that there can only be one youngest brother, so it is unnecessary to actually name him. By stating that the brother is the youngest one, we already know who the speaker is referring to.
First of all, we must identify the appositive phrase: an appositive phrase is two or more words standing as a single unit without a subject or a verb, whose function is to rename, re-identify or describe a noun placed right beside it. In the sentence, “Julian” is the appositive because it renames the noun “My youngest brother.”
This appositive is nonrestrictive <u>because, in contrast with restrictive appositives, it is not essential to the meaning of the sentence (If we had to remove “Julian” from the sentence, it would not affect the sentence's meaning) and it is enclosed by commas.</u>
