Brilliant! And then -- it whacks me with the adult problem.
When the main character is a child (straight up to the age of majority), unless the story deals with age-appropriate problems that that kid ought to learn to cope with himself, adults have to be absent, evil, or incapable. And while I can make the aunt rather young -- only seven years older than our heroine, I was thinking within the realm of possibility, which would make her closer in age to the girl than to her sister, the girl's mother. . . that's enough of an age difference that the older girl has to play the adult. At least enough to stifle a story. Without really careful manipulation, because pushing her off stage, making her evil, or limiting her ability to act would throw the whole thing out of kilter.
I think this story is going to have to take more time than I had thought. For a woman in her early twenties to let a teenager help is a different thing than a teenager to let a girl. Also -- and in hindsight, this would have been necessary if they were both girls -- the villain of the piece is going to forcibly prevent their getting help from the more mature and capable characters. Fortunately, she hates them both personally.
(Meanwhile, I've still got to give the girl a name that readers will think a doting mother would bestow on a girl she wants to think of herself as especially special, but which, either in itself or as a nickname, she can bear the rest of the story.)