Is the movie The Exorcist ok for a 12 year old?

I am 13 but my Sis is twelve. I love horror movies but The Exorcist is the one movie I was told would really mess with ur mind. My Sis is goin to a sleepover tomorrow and they r gonna watch it-- I'm worried she mite get nightmares.. Am I nervous for no reason? Is it ok if She watches it??