Family members in the singular don't have the definite article. So it's Mio padre, mia madre, mio zio, mia zia etc. etc. BUT in the plural it does. i miei genitori i miei fratelli le mie sorelle. http://italian.about.com/library/fare/blfare124a.htm It's just one of those quirky things.
The rule -When the possessive adjective refer to family members in the singular, you cannot put the article- has some exceptions:
la mia mamma - (my mom)
il mio papà - (my dad)
la mia sorellina - (my little sister)
la mia cara zia - (my dear aunt)
It's the exception to the exception as pierugofoz said. See http://italian.about.com/od/grammar/a/italian-possessive-adjectives.htm for details