NO NO NO NO NO
It's a misdirection joke. The set-up was intended to suggest he is a father, but the punchline is simply that he is holding a baby. I think it's a first. He would never hold his little sister, who is almost exactly 4 years younger, because he was afraid he would "break" her.
And you're right, he does look 12 (he's 19), but that's a family trait. I am 53, but barely look 52.