"My father is inside."
We use very often the article in front of possessive adjectives BUT very seldom when speaking about relatives. There are some exceptions, when you specify something more because you have more relatives of that kind.
"Mia cugina è alta" BUT "La mia cugina più alta è Lara"
"Mio fratello si chiama Gianni" BUT "Il mio fratello più giovane ha appena cominciato l'università."
There are some cases where both with and without article works... I don't think it's worth mentioning, if you need some more details I will find some resource in the web. :)