These words are implied: The boy [who is ] talking to Angela is her younger brother.
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hanuman_2000So it is not an adjective.The whole clause is adjectival but the word itself is not.
hanuman_2000The 'wearing blue jeans' is a participle phrase modifying the 'girl'.or 'who is(implied) wearing blue jeans' a relative clause modifying the 'girl'.'Implied' does not affect or determine the actual grammar.
hanuman_2000I would like to know whether it is a reduced relative clause or participle phrase.A reduced relative clause IS (one kind of) a participle phrase/clause.
hanuman_2000participle phrase or participle clause?Both terms are used. More recently "clause" is more often seen.