As you all know Quicksilver and Scarlet Witch are best known as the children of X-Men villain Magneto, and Quicksilver as some X-Men fans know is often the leader of the Brotherhood of Mutants, and Scarlet Witch is sometimes more of a different story from the two.
Even though the siblings are mutants, they both have became official members of the Avengers and Scarlet Witch was married to Vision. And when they both appear in the film, it will be the start on having the two X-Men characters act as a major part of the Avengers.
It was also because of Coulsen having a cellist girlfriend that fans thought that he would be ressurected as the Vision but now it all seems false as Coulsen is ressurected for Agents of SHIELD and with both Scarlet Witch and Quicksilver considered to be young avengers as Marvel has eyes on Saoirse Ronan on playing Scarlet Witch.
Joss Whedon confirmed the brother-sister duo will appear as new members but also said that they won't make things easier for the Avengers that they will cause trouble.
Well not to worry, as rookies they will first have difficulty fitting in but later on they will be sure to make things right on helping the pros take on the Masters of Evil, the Chitauri, and most likely the newly reintoduced Mandarin.