Where's the sister?

Yesterday we visited our friends Brad and Cari and their brand new baby girl who was only 1 week old. I couldn't believe how tiny she was. Next to her, our kids seem enormous and I can't even remember them being so small and birdlike.
M seemed to be both slightly afraid of and fascinated by the newborn at the same time.
She asked a lot of questions:
M: "Did the baby come out of the mommy?"
Me:"Yes, she did. Remember how we talked about how mommies push their babies out of their bodies when they are ready to be born"
M: "Oh. But where's the sister?"
Me: "The sister?"
M: "Yes. Where is she? Is she coming out soon too?"
Me: "No, sweetie. They only have one child."
M: (incredulous, turning to our friend Cari) "You only have ONE child?! Why? When are you going to have another one?"