It's a 80's movie I think and it starts with the a younger brother eating chocolate and getting it all over his older sisters diary and the sister comes in the room and the little brother says "hello louise"

There's a scene similar to that in Teen Witch (http://www.imdb.com/title/tt0098453/)
YouTube (http://www.youtube.com/watch?v=GU6vCugzm3E&feature=related)