Marvel has the animated rights to Wanda and Pietro since they are both recurring characters in the "Wolverine and the X-Men" series, which is made by Marvel Animation.

While it would make sense to me that they might keep the X-characters out of the Avengers cartoon to make it clear they are separate universes (obviously Superhero Squad doesn't have this problem), Wolverine and the X-Men hasn't been renewed for a second season, leaving the X-characters in animation limbo for now. So depending on Marvel's animation plans for the X-characters, the odds of Wanda and Pietro appearing have increased I'd say.

That being said though, since Avengers: Earth's Mightiest Heroes is clearly based on the movie universe, that probably hurts their chances since there is no way to put them in the films.

~Hat~