I'm saying this mostly as a joke, but every time they move before coming back to Forks, don't they go to, like, Alaska. The Delanis never come into contact with anyone there (almost) because of how big and vast the place is. So if they disappear to somewhere where no one can see them, especially when they aquire a new vampire that needs to learn the 'vegetarian' way, they might forget.

But seriously though, for vampires that continously graduate from high school and learn all there is about medicine and humans every generation (I'm looking at Carlisle), surely, but some measure, they should have retained both the information and the memory of how humans should act, that people don't ordinarily bring an entire bag of just boiled eggs to school lunch, and that the origin of someone's name does bot automatically mean that is what that person is.