Marvel wasn't kidding when they've planned until 2028.
Scarlet Witch and Quicksilver are miracles, not mutants.
Fox currently owns the movie rights to the word 'mutant' so the question was 'How is Joss Whedon going to use Magneto's children in a movie without mentioning their parentage or species?' Turns out the answer is 'Ignore it,' which is a valid move. There were decades of Scarlet Witch and Quicksilver stories before they were revealed to be the children of Magneto because comic books are nothing if not soap operas in spandex. And like many zombie movies and shows that exist in universes where the word zombie was never coined, miracle certainly seems an accurate description of the superhuman abilities manifesting in these people.