Some key notes:
We can make is so that the girl is being tempted or we can do a world driven thing - you need to become the next babayaga to restore order.
Multiple Endings - the girl becomes babayaga, and she stays that way, or the boy saves her from this fate.
If the boy wins does he defeat babayaga?
If the girl wins does she go with babayaga, and the boy goes home?
The boy's and the girl's relationship is echoed in the world around them.
Even if the girl cannot become the next babayaga... something... something.
Maybe game mechanics can be so that the AI is always getting in the way of the other character - so when you have a choice in the story to get them out of the way, you can actually get rid of them?
Once they get their ending, do they go back and play from the beginning? We can make this decision later.
Once upon a time there was a brother and a sister, Elise (10) and Ivan(8).
Everyday they lived with their father, an officer in the army.
Everyday, Elise looked out the window and dreamed of the day she would be able to explore the world.
Everyday, Ivan was a busy body that always sought to be helpful in the house (running errands for the cook, for the nanny). He was also kind of annoyed that his sister never tried to help out much.
Until one day, Elise finds that their shed has gone missing. She wakes her brother and they both go to find it. They find the shed on a hill. When they go into the shed, babayaga's property (with the shed on it) stands up, and the property takes off into the night.
Think: how the story should progress and how we can make all endings satisfying (but two of them kind of, still, unsatisfying).
We should make babayaga interesting enough that the girl finds babayaga fascinating, but creepy enough that the boy would want to escape.