Answer: The appositive phrase in the sentence given is:
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“little Sierra” .
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Explanation:
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An “appositive” refers to a “noun” or “noun phrase” that renames another noun right beside it.

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In the sentence given: 

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“My neighbor, little Sierra, threw rocks through her parents’ window.” ; 
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the phrase: “little Sierra“—renames the noun: “neighbor“.

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As such, the phrase: “little Sierra” is an ‘appositive phrase‘.

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