Previous Post ...is this official clarification of the status of Quicksilver and the Scarlet Witch re future Avengers movies: http://blogs.coventrytelegraph.net/thegeekfiles/2012/04/comic-book-bites-scarlet-witch-and-quicksilver-could-appear-in-avengers-and-x-men-doctor-strange-and.html Anyway, Avengers took £17.5m on day one in the UK, surpassed only by Star Wars 3 (Sith) and Harry Potter and the Order of the Phoenix. It's looking possible that there'll be an Avengers 2.

This further explanation came out today:

Another update on Scarlet Witch and Quicksilver and the deal with FOX.
FEIGE: We both have them. There's a specific arrangement with those two characters that would allow us to use them with "Avengers," but not discuss or reference their mutant or Magneto-related lineage. They can use them as mutants and as Magneto's relatives, but cannot have anything to do with "The Avengers."

So they seem to have an easier "in" on the X-men side of things, where as the Avengers can't really reference anything about their existing past or where their powers come from. I suppose they might be able to hint at it, while leaving the specifics out however.