Well...when a mommy and a daddy love eachother very much and want a daughter, they call the stork!

Anyway. In one of Zuko's memories, and you see her in the audience near Iroh when Zuko's father burns him and exiles him. [The Storm ep] You see a vague picture of Zuko and his younger sister. Then in the last ep of Season 1, his sister first appears. Thanks all. Now his sister wants to throw him out of existance for the throne.