You were Harry's sister and you were Fred Gideon Weasley and George's bestfriend since first year.

You were at Fred's dorm because you did a pyjama party in here with the other Gryffindor and George,

You accidently left your bag in his dorm where you got your diary in it. When you came back in Fred's dorm to catch your bag.. There he was.. with your diary in hand, smirking, his famous grin.

You know you were screwed because in the diary you writed something about Fred.