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."