And then the muse, usefully, starts playing with the notion of how to make it as painful as possible. If the mother was, in this world, the equivalent of a Squid, it would be even more bitter than if only her sister had had magic. Though there's also the question of whether she actually can't do magic, or was just rejected as a student (which, of course, might have been told her as "she can't do it" to keep her less dangerous).
Whereupon I finally get some progress: she is going to find a way to do magic. It involves things that everyone would be happier if no one did them, ever.
At which point I notice that the focus has changed to her. Also, that I have a problem in that she would be an unpleasant person to follow for a story. . . hmmm, perhaps it's one of the adults who take the girls who is the main character. Or at least it can be a multi-point-of-view story.
And I may have to change the girls' ages, because they should not be so young that the adults would shield them at all costs.