Is "aunt" a proper noun?

No, aunt is not a proper noun.

Proper are specific. They are the names of things, people, etc. so they start with a capital letter. Example: Many Filipinos vote for Duterte.

But just because it starts with a capital letter, it doesn't mean that it is automatically a proper noun. Example: The Orwellian culture is quite interesting. Orwellian here is used as an adjective. We know that because it precedes a noun which is culture.

Proper forms of words start with capital letters. You just have to know whether it is a noun, adjective, etc.