The 50-year-old actor's wife Hilaria gave birth to Carmen Gabriela - the couple's first child together - three weeks ago and he has admitted she hasn't been introduced to her older sibling, from his previous marriage to Kim Basinger.
Speaking on 'The Late Show with David Letterman' about the age gap between his new baby and Ireland, 17, he said: "It's weird, you know?"
He added: "We're going to hand her this three-week-old baby and say, 'This is your sister.' "
The day after Carmen Gabriela was born, a proud Ireland posted an open letter to her half-sister.
She wrote on Tumblr: "Dear Carmen, You have entered the world! Welcome! We have all been anticipating your arrival! By we, I mean a lot of people.
""There are countless lessons that I want to teach you, but I have faith that you will learn a majority of them on your own journey and they will help you grow."
She continued: "I love you, Carmen. You have been born into one crazy family.
"You are both lucky and cursed at the same time. We are going to have so much fun together. I can't wait to meet you soon."