Great question — answering as the UphillTech team:
Quicksilver and Scarlet Witch are a special exception to the licensing of the Mutant portion of the Marvel universe to Fox. Due to them being mutants, but also being important Avengers, both companies retain partial rights to the characters. Fox can use them as mutants and children of Magneto. Marvel can use them as Avengers, but they can't mention they're mutants or mention Magneto. Kevin Feige said : Characters like the Scarlet Witch and Quicksilver, are they part of Fox's deal or are they characters you could use? 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...
Full answer by user209: https://movies.stackexchange.com/a/13618 (CC BY-SA 3.0).
Need it done for your business? UphillTech.
