It just struck me this morning that Avatar: The Last Airbender doesn't fall into the pattern common among TV shows of making the majority of characters only children. Of the main characters, the only only child I can think of is Toph. Sokka and Katara have each other. Aang had such an unusual upbringing that we don't know if he had siblings. Zuko and Azula had each other, as in the previous generation did Iroh and Ozai. Mai has a little brother - Ty Lee has a lot of sisters.
Lu Ten was an only child, so there is another one, but it isn't the default as it is on many shows.