Gale takes away Prim, when Katniss declares, "I volenteer for tribute." Why did she want to fill in for her sister? You can't possibly volenteer for somebody when they asked you.

Oh, I didn't know Aslan.

This question has been asked before. Katniss knows her little sister is weak and would not last in the game more than a few minutes. At least Katniss has hunting skills and is tougher than her sister. She loves her sister too which is the main reason she volunteers.