Answer:
Appositive: my youngest sister
Noun or pronoun renamed: Mary
Explanation:
An appositive is a noun or noun phrase set next to another noun or pronoun to explain or identify it. When it's essential to the meaning of the noun it refers to, the appositive isn't framed by commas. When it isn't essential, like in the given sentence, it is framed by commas.
The appositive in the given sentence is my youngest sister. The noun renamed by it is Mary.
