Revealing mistake: When Katniss is blown back from the explosion, her stunt person is visible. This can be proven because the stuntperson's skin tone is different and her hair is not in a braid. It is in a ponytail. Wires can also be seen pulling her to give the illusion that she was blown back from the explosion. (01:33:00)
Revealing mistake: Katniss shoots and kills someone who throws a spear at Rue. When Rue says "you have to win" we see a shot of the dead bloke with the arrow sticking out of him. In this shot, if you look carefully you can see a square outline under his shirt that the arrow is stuck in. (01:36:20)
Revealing mistake: When Katniss is running from the fire bombs, when she lunges forward, you can see her stunt double's face.
Answer: Besides Prim being her younger sister, when their father died, her mother basically shut down out of depression, leaving Katniss to raise Prim for the most part. She has a bond that goes beyond what some sisters might normally have.
Bishop73