A message from my oldest niece

To day I received a postal priority pack from my sister. Inside was a birthday card and pictures from her family, a drawing from her younger daughter, and a message from her older daughter:

Qrne hapyr gbz,
unccl oveguqnl! Naq n zreel
puevfgznf. Ubj ner lbh? V nz
tbbq. Ybir,
Pngurenar

While I visited her for Thanksgiving we exchanged a secret code so we could send messages to each other. She had a code she used with a friend, but it was too complicated and they don't use it (every letter had a different symbol. Have you ever tried to create 26 unique symbols?). So, yes, I taught her ROT13.