What girl from District 12 is chosen chosen to attend the Hunger Games why is it ironic that she is the one who was chosen

Katniss's younger sister, 12 years old. She goes by "Prim." Prim is much more innocent and child-like than Katniss, and not just because she lacks the hunting aptitude of her older sister. It is ironic because Prim is the least able to deffend herself in the games.