Than's sister

Tahn's sister was a refugee who was journeying with her brother and his pregnant wife toward Ba Sing Se.

History
Nearly nothing is known about Tahn's sister. All that is known is that at some time, she was forced to flee her village after the Rough Rhinos attacked along with her family, Tahn, Ying, and the then-unborn Hope.

Traveling up the west coast for months, she met Team Avatar near the waterfall pond. Both groups decided to travel together toward Full Moon Bay where they would take the ferry towards the Earth Kingdom capital. After they had their passports stolen, Aang agreed to lead them through the Serpent's Pass in order to get them to Ba Sing Se safely.

While crossing the pass, a giant sea serpent for whom the pass is named, appeared out of the water, threatening her family, as well as Team Avatar. Aang, with the help of Katara, defeated the creature and guided the family through the remainder of the pass without trouble. When reaching the end of the pass, Ying went into labor and Tahn's sister was present during the delivery.

When traveling on to Ba Sing Se, Aang noticed a titanic drill, threatening to break through the wall of Ba Sing Se. With the help of Toph, Aang brought the entire family up to the top of the wall with earthbending. After the destruction of the drill, Tahn's sister and the rest of her family, took the Ba Sing Se Monorail toward the Lower Ring of the city where they took refuge. On this train ride, she encountered Iroh and Zuko, although she wasn't aware of their true identity.

Relatives

 * Than (brother)
 * Hope (niece)
 * Ying (sister-in-law)

Trivia

 * Though she appeared in nearly every scene in "The Serpent's Pass", as well as a couple of scenes in "The Drill", Tahn's sister had no lines and was never even formally introduced. Her relationship to Tahn, Ying, and Hope is known only through "Avatar Extras".
 * Although she traveled with her brother and his wife, she wasn't seen when Zuko pondered about stealing Tahn and Ying's food while he was traveling alone.