What song is it that Lizzie Pattinson(Robert's sister) sings in on the soundtrack? Cathrine Hardwicke said that she sang and I cant figure it out.

It's called "Who Are They?" on Twilight:The Score.
The Score is the musical compositions in twilight by Carter Burtwell, no lyrics, just music. It's very good. I recommend buying it

She is heard on the Carter Burwell track "Who Are They" when Edward Cullen enters the cafeteria.