You should creatively create a costume, you can with very little money. Don't be lame.
Yea, just put n a cowboy hat and plaid shirt and jeans
Pull your t-shirt over your head, poke a couple of eye-holes in it, and go as a ghost.
Get identical white teeshirts for you and your date. Label one "Peter" and one "Pumpkin" You'll be the hit of they party.
(Might not be good if this is at a church or something similar.)
Yes go,,just wear ordinary clothes,,and be you're sister,,hahahah.
Write 'generic costume' and draw a barcode on White t shirt. Problem solved.