Who is related to who? (джен)

Toph was twelve. A group of four random kids showed up in her lawn asking her to train the avatar. A fifth person stayed nearby but never revealed themselves. Time passed and only one question was left on her mind : Who is who's brother and who the hell is who's sister?
Azula was six. When she showed up on the ice. No one knew for sure where she was from or who she was. She refused to speak of it. She ended up being unofficially adopted by two children's Gran Gran. The three of them - Katara, Sokka, and Azula - grew up as siblings.
Zuko was fifteen. When he arrived at the Southern Water Tribe. He had one shot at fixing everything that had gone wrong in his life. And it was to find the Avatar. He was responsible for his little sister's death, so if he should die on the journey it would only be fair.
Imagine how he felt when the same little sister who had fallen into the Arctic waters at the age of six, stared up at him from the snow with anger in her eyes.