Misconception

I noticed several people worried that Quicksilver and Scarlet Witch couldn't be mutants in Avengers 2, first off neither Fox nor Marvel own the word Mutant, it is technically a biological term and has been used in various sci-fi stories since before the X-men were created so there is a good chance they can still be mutants. Even if by some legal wrangling Fox figured out a way to prevent them from being referred to as mutants there are many ways around this.

For instance the term mutation is up for grabs so they could explain that they were given their powers through genetic mutation and be done with it. As for the Magneto thing, it is very easy to deal with that. Don't bring him up, there are many people with far better relationships with their parents than they do with Magneto who rarely ever bring up their parents in conversation unless a holiday is coming up or they happen to get a call from their parents.

Given their relationship with Magneto its entirely possible that they wouldn't even want him brought up. They could honestly fix the issue with a line, just call him a terrorist and have one of them say they don't want to talk about him.

Not saying I like the idea of not bringing up Magneto at all just saying its entirely within the realm of possibility that they just wouldn't bring their parents up.

I think they could totally retcon their parentage in the movie-verse and say that it was Whizzer and Miss America's children like they were originally thought to be in the comics. It was many years before it was discovered that Magneto was their father (a retcon itself really.) So I think they're going to have a connection to some other "super-soldiers" from Cap's time during WWII.

They don't have to work from the grey. They could easily make it without having to mention anything about mutations or Magneto. Not yet, at least. I don't think the fans would care. If it's Avengers, then no one's going to demand a detailed backstory from two random characters. If they got their own movies, or were put in X-men, then perhaps, but it's not necessary. I think "mutation" could work. Hint at the X-men if they have to.

There's no need to mention where their powers come from who who their parents are.

It's irrelevant to the movie, only important to the comics.

The point I was trying to make is that if they decided to go with a more true to the comic route it would be relatively easy to work around the copyright issues. Basically their agreement with Fox says that they can use them so long as they don't mention Magneto, The Brotherhood or the X-men.