Legally, the Avengers movies can use the characters, but they can not be mutants nor can they be related to Magneto in anyway. Unless Marvel gets the X-Men rights back, they will be just shadows of their characters rather than the characters are suppose to be.
Not necessarily. All they legally have to do is not specifically mention "mutants" or "Magneto". In the world established they hardly have to use the term Mutant to have their superpowers make sense and saying they or their Father were know terrorists covers that part of thier origin fine.
As long as the pair are close siblings, Pietro is an arrogant super fast egoist who tries to protect his sister, and Wanda is a formerly sheltered slightly strange young woman with the ability to make things go catastrophically wrong, you've got all of them that matters characterwise.
ZacWilliam, can't think of any other siblings he could mean either...
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