Fox owns the right to "mutants," but not all the specific characters themselves, so Scarlet Witch and Quicksilver are getting in through a loophole by not mentioning that they're mutants, and not mentioning Magneto as their father. SW and QS are pretty much confirmed last I heard for the next Avengers movie, along with Falcon, so there you go.
If you're interested in Rule 63'd characters, there's plenty of them in the books. Spider-Man has Spider-Woman and a ton of Spider-Girls, Wolverine has X-23, Iron Man has Rescue (though she's out for the time being), Stiltman has Lady Stiltman ("It's a homage!"), so on and so forth. Most of them are similar enough to be recognizable but unique enough to differentiate and stand on their own. Sometimes.
Edited, Jul 9th 2013 10:05am by lolgaxe