There is no unclear reference in the sentence. The only pronoun in the sentence (they) clearly refers to 'this school district'. Perhaps the problem with the sentence is that a plural pronoun (they) is taking the place of a singular antecedent (this school district). However, it is often acceptable to use a plural pronoun to take the place of a singular noun that is a word for a number of people or things. Examples: The staff is off today. They are at a seminar. The team will be a little late. Their bus broke down. The herd got out of control. They trampled the corn field.
The phrase in this sentence is Nonrestrictive because there can only be one youngest brother, therefor the name Julian does not need to be in the sentence, because it is unnecessary to name him.
