The relationship between these two individuals is that they are siblings. Specifically, Katniss is the older sister and Prim, the younger sister. Katniss is clearly very protective over Prim and wants nothing but the best that their limited resources can give her. She volunteered in her sister's place to keep her from harm and to keep her an innocent and young for as long as possible. These sisters love each other very much and that is plainly visible in their actions, attitudes, thoughts, and concerns.
