It’s confirmed that the mutant siblings, Scarlet Witch and Quicksilver will be in the upcoming Avengers 2 movie.
The two characters were previously hinted by Joss Whedon, but Entertainment Weekly has confirmed the characters will be in the ranks of Avengers heroes. However, Marvel Studios is still holding things back and states that the script might change in the next six months.
Apparently, things would get tricky too if the characters stay in the script. The characters are first introduced in X-Men, and they are the kids of Magneto and later become members of The Avengers. While Marvel can use these two characters, they cannot use or mention Magneto, X-Men or even the word mutants, because Fox owns all the screen rights of X-Men. Good luck trying to explain their powers, and their origins.
Personally, I really like to see these two characters making appearances in the movie. But if they’re making too many changes to the characters, they’re hardly the same characters anymore. We will see eventually what they done to the characters.