Mom and dad, your brothers and sisters, grandpa and your Aunt Millie. They're all special people, and so there's a rule just for them. In the plural form, the Italian definite article will appear before the Italian possessive adjective that refers to a family member or relative. For example, instead of saying "my brothers," you are literally saying "the my brothers."
The table below contains other examples:
USE OF ITALIAN DEFINITE ARTICLES WITH POSSESSIVE ADJECTIVES
Mio fratello è carino.
(My brother is cute.)
I miei fratelli sono carini.
(My brothers are cute)
Questo è tuo zio.
(This is your uncle)
Questi sono i tuoi zii.
(These are your uncles.)
Mia nonna è vecchia.
(My grandmother is old.)
Le mie nonne sono vecche.
(My grandmothers are old.)
Mio cugino è straordinario.
(My cousin is exceptional.)
I miei cugini sono straordinari.
(My cousins are exceptional.)
From:https://www.dbfei.com/Article/yidaliyu/201311/7726.html