An appositive phrase is a noun or a noun phrase that "renames", identifies or gives further information of another noun placed right beside it. This type of phrase can be either restrictive (when it's essential to the meaning of the sentence) or nonrestrictive (when it's not essential to the sentence, and therefore should be set off with commas, dashes or brackets).
In the sentence, "a golf pro" is an appositive phrase because it is placed right beside the noun: "Raoul's sister", and it gives more information about that noun. Furthermore, it is nonrestrictive because it is not essential to the sentence and it's enclosed with commas.
