No they have complete rights over Scarlet Witch because she was a Brotherhood member.
My main problem with the film version is aside from my hatred of the hairstyle and costume, it's just well if Marvel Studios are using those characters why the hell would I want to see Inferior versions done by Fox? Especially when they could use characters other studios don't have the rights to such as Nightcrawler and Polaris.
Also isn't it crazy how Quicksilver has suddenly become someone who two studios are fighting over and for the next 2 years he's going to be in Movies 2015 he's in Avengers and 2016 he's in Apocalypse.
I would love that actually, Quicksilver should be in both the X-Franchise and the Avengers because he can literally be on two teams at once no explanation required and because of his friendships with several X-Characters. But I see your point about the X-Franchise too many of my favorites there get shafted just so we can get more Wolverine and Cyclops having a Dick Measuring contest.