I don't know man. I think it's a pretty safe bet that these characters are supposed to be Quicksilver and the Scarlet Witch. Granted, the Marvel Cinematic Universe is growing each year but at the end of the day there is only so much room in the Avengers films. They are going to go with fan favorites instead of obscure characters.
especially with Feige making a point to say that they can actually use Wanda and Pietro... it makes complete sense that Whedon is talking Wanda and Pietro... i know people have suggested T'Challa and Shuri, but Shuri isn't that well known... Wanda and Pietro have been connected to the avengers longer than the x-men in a way (especially Wanda)