妹妹 or 弟弟

Little O: Mommy, di di. Mommy di di.
Me: What is di di? I don't understand you Owen.
Little O: Mommy, di di.
Me: Owen, do you want to have a 妹妹?
Little O: Nooo...(shakes head)
Me: Owen, do you want to have a 弟弟?
Little O: Yah...(nods head)
Me: But Mommy wants a 妹妹.
Little O: Mommy 妹妹. Owen 弟弟.
Me:....