I think they should have a twin brother, but the second twin is not related to them. that the twins came from different eggs. But their whole life they have been told they look alike when they don't.
Twins is a great idea, really got my brain ticking. Maybe they go to see a sibling on the list and discover there's two but only one was put on record because of a clerical error putting the sibling count up to 28.
Dawn would be made up and keep using the phrase "Two for the price of one" and also "two birds with one stone" to them. Dawn could also be really interested to find out about their phychic ability (which they don't have) Dawn could be phy-curious. I think most twins get quizzed about this I think they get pretty tired of it too.
Where's your brother gone?
I don't know
(Amazed)What!? You don't know where your own brother is?
I can't believe that! You don't actualy know, where your own brother is? But you're twins!
Twin just stares at Dawn lost for words
Chuck in a few famous twin references "The Weasly's" "The Olson's" "The Mcdonald's" and there you go.