In all of the books of the Hunger Game series Katniss does everything she can to protect Prim. When Prim's name is drawn at the reaping she volunteers to take Prim's place as tribute. President Snow threatened her family and she did evrything she could to protect them. When she finds out she will have to fight in the Hunger Games again she knows she won't live through this one because she vows to herself that she will protect Peeta no matter what because he saved her life in the 74th Hunger Games. Peeta is also determined to do anything he can to make sure Katniss comes back alive. When Haymitch's name is drawn he immediately volunteers as tribute so that he can protect her from within the games.