The Avengers 2 features Scarlet Witch and Quicksilver, though Marvel Studios is not allowed to name them as mutants or reference their father, Magneto, due to Fox Studios owning the rights to the X-Men.
Now that may not be a problem as Marvel Comics has apparently retconned the relationship between the trio.
As Newsarama points out, the latest issue of Avengers & X-Men: AXIS #7 by Rick Remender and Adam Kubert sees Scarlet Witch cast a spell striking anybody in her family down - with Quicksilver and Magneto present.
Well, Quicksilver takes a dive, but Magneto is left standing - much to the surprise of Scarlet Witch - who screams it's all been lies, and then laughs at Magneto for being alone. Connecting the dots, it's obvious that Magneto is not the father of Scarlet Witch and Quicksilver.
It's still unclear how Joss Whedon will be explaining Scarlet Witch and Quicksilver's powers in the Avengers 2, but if this Marvel comic is any indication to go by, it won't be that they are mutants (obviously). It also won't be that they are Inhumans, as previously Marvel Studios president and producer Kevin Feige stated there is no connection. Some are speculating Loki may have given the twins their powers or even an Infinity Gem.