Regarding the twins, in the comic book storylines, Mephisto gave her some of his life essence, or something like that, to help her create them. And they stayed real for three years before he decided he wanted his essence back, reabsorbed it and the kids stopped existing. Then in another storyline, they were somehow reincarnated and existed again for good and became the adult Wiccan and Speed and were part of the Young Avengers. A group which Marvel seems to be setting up for the MCU. Now, Mephisto may not be involved at all and the kids will just poof away when this alternate dimension bubble thing collapsed, pushing Wanda over the edge. One other thing that various articles have discussed, though, about her powers, is that she has primarily been telekinetic and not magic, so I do not know how they will show or explain the shift in what she can do.