The proper noun in the sentence "I told Aunt Bess that my brother had won a special award for his research," is Aunt Bless.
Proper nouns have two distinct characteristics. They begin with capital letters, no matter in which part they appear within a sentence, and they name specific one-of-a-kind items.
All nouns are words which name places, things, people, animals, and ideas.
In this case, the proper noun refers to the name of a person.