Prim misses Katniss and is writing mind letters to Katniss. If you ever noticed, whenever Katniss thinks of Prim, she gets pain. Maybe she's getting the mind letters. Think about it.
Prim's POV
She's not gone. Just on a trip. I'll write a note to her.
Dear Katniss.
Katniss, don't go. I miss you already. Who's going to take care of me? Or help me with my homework? I want you back. Mom blacked out as siin as we got home. I can't take care of myself. I don't want Gale to help me. I want you to. Help me.
Love,
Your Little Duck