I believe it is Star Wars, because you are adding on that you are watching that particular movie. The sentence would have been fine if you kept it its original way without the ",Star Wars." (We are watching one of my brothers favorite movies).
The words in the sentence that make up the appositive phrase are โStar Warsโ. An appositive is a noun or noun phrase that renames or gives additional information about a noun next to it. In the sentence โWe are watching one of my brother's favorite movies, Star Wars,โ the phrase โStar Warsโ provides more information about which of the brotherโs favorite movies they are watching. Thus, it is the appositive, renaming the movie that is being watched.
The appositive phrase in the sentence is "Star Wars," which renames the noun phrase it's associated with. An appositive provides additional information about the noun or noun phrase. It is specifically used to clarify or specify which movie is being discussed in this context.
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