Thresh's sister is a citizen of District 11. She is presumed to be raised by her grandmother along with her brother, Thresh. She was only mentioned on the 74th Victory Tour. Katniss was certain that she was shot, after hearing gunshots that she knew killed an old man, but was proven wrong by Seeder, as when Katniss asked Seeder before the start of the 75th Hunger Games, she answered: "they are alive." Her current fate is unknown.
The only thing known about her appearance is when Katniss describes her as tall and muscular, just like her brother. (book)
Thresh's sister is not shown in the film for Catching Fire, only whom [Katniss Everdeen] assumes to be his grandmother.