In the sentence "Some of my cousins, whom I had not seen for several years, came to the reunion", the adjective clause is: "whom I had not seen for several years" and it modifies "some of my cousins" -the subject in the sentence-.
Adjective clauses can be identified because they start with relative pronouns (such as "whom", "which", "that") or a relative adverb ("where", "why", "when"). These clauses also provide further information (I had not seen some of my cousins for several years) about the subject ("some of my cousins").
