Leave it to Joss Whedon to launch a nerd-baiting rumor fest with 22 words dropped into the middle of a normal interview at someone else's movie opening.
Crave's CinemaBlend website posted video of the interview at the premiere of Iron Man 3, in which Whedon, writer and director of Marvel's The Avengers and the sequel due out next summer, says, "I've got these two characters, two of my favorite characters from the comic book, a brother-sister act, they're in the movie."
Sure, there are a lot of siblings in the Marvel universe, but Wanda and Pietro Maximoff (Scarlet Witch & Quicksilver) are long-time Avengers in the comics and may make a good fit for both Whedon and Marvel. (Marvel has made overt references to cross-over movies and Scarlet Witch and Quicksilver are both mutants and Magneto's children.)
Whedon's comments come about 2:00 into the interview below. Watch it and see if you can read anything into his answer or his body language.