Than's sister

Tahn's Sister is a refugee that Team Avatar meets during the episode, "The Serpents Pass." She has no lines in the series and was never formally introduced. The only thing that is known about her is that she is a refugee who was traveling to Ba Sing Se in order to be safe from the Fire Nation

History
Nearly nothing is known about Tahn's sister. All that is known is that at some time before the episode, "The Serpent's Pass," she was forced to flee her village along with her family, Tahn, Ying, and the unborn Hope.

Meeting Team Avatar
In the episode, "The Serpent's Pass," Tahn, Ying, and Tahn's Sister meet Team Avatar. After they have their passports stolen, Aang agrees 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 appears out of the water, threatening her family, as well as Team Avatar. Aang defeats the lizard and guides the family to Ba Sing Se, where he notices a titanic drill, threatening to break through the wall of Ba Sing Se.

As Team Avatar destroys the drill, Tahn, Tahn's Sister, Ying, and the newborn Hope enter the city, also encountering Iroh on the way in.

Fall of Ba Sing Se
Her fate after Ba Sing Se fell to the Fire Nation is unknown, as is her life after the war as a whole.

Trivia

 * Though she appears in nearly every scene in, "The Serpent's Pass," as well as a couple of scenes in, "The Drill," Tahn's Sister has no lines and is never even introduced; her relationship to Than, Ying, and Hope is known only through Avatar Extras.