Lee's been explaining to Mrs Weasley. It's punishment. I think it's like Pansy's, only instead of saying awful things, they've been made to think the other twin is dead.

Lee's with them. I haven't had a chance to talk to them yet, so that's all I really know. Ron, are you with them too?

And I think they switch journals sometimes. They pretend to be each other all the time. So maybe Fred was pretending to be George, or had his journal before they got punished.