This hasn't been confirmed, but I was under the impression they couldn't be due to legal rights with Fox. According to IGN, this might not be the case:
"Yes, they originated in the X-Men comics, which is generally 20th Century Fox's purview, but it appears there's more to it than that for the brother/sister duo, who went on to become Avengers in the comics. Instead, Feige reveals both 20th Century Fox and Marvel have legal access to Quicksilver and the Scarlet Witch in their respective films, due to the unique nature of the characters. That's right, Marvel could put them in an Avengers movie, while Fox could also separately put them in an X-Men movie if they so chose to."
Seems pretty interesting to me that we could have the brother/sister duo in the next Avengers movie. Thoughts?