The former '30 Rock' star, 55, and his wife Hilaria welcomed their first child, Carmen Gabriela, into the world on Friday, and his 17-year-old daughter Ireland, whose mother is Kim Basinger, hopes they will be best friends when they are older and Carmen will learn from her mistakes.
The self-confessed "experimental, wild child" posted a letter full of advice to Carmen on her Tumblr site just hours after her birth, saying: "You have entered the world! Welcome! We have all been anticipating your arrival! By we, I mean a lot of people. Girl, you haven't even met them, but there are a ton of people who care about you."
The blonde model warned her newborn sister about drinking, smoking marijuana, dating, trusting the wrong people, and assured her that she will always be there for her.
Ireland also addressed her own parents' bitter split and how much it hurt her growing up.
She wrote: "Remember your parents will always love you. As a kid, you forget that sometimes. When they are screaming about this and bickering about that. It hurts. Grown ups yell. I don't know why, but they do. No matter what your mom or Dad says or does, simply remember that they love the s**t out of you. NEVER forget it."
She added: "I love you, Carmen. You have been born into one crazy family. You are both lucky and cursed at the same time."
Alec and Hilaria are "overjoyed" after Carmen's birth.
The 29-year-old yoga instructor tweeted on Friday: "We are overjoyed to announce the birth of our daughter Carmen Gabriela. She is absolutely perfect (sic)."