The dictionary doesn't really help, as I don't understand how any of the meanings really apply here:
- to dedicate; to make an offering; to pay (fees)
- to supply
- to store
- to finish; to bring to a close
- to restore (something to its place)
- to achieve (e.g. a result)
The closest definition seems to be 4 or 6, assuming that the word doesn't have some special meaning related to "acting", since "heroine" and "villainess" are 'roles' of sorts.
My best guess is that it means: "(From those memories,) I learned that, in this world, my younger sister Mishuly would become the heroine at the end of the story."
As in, she either brings the story to a close as the heroine, or she ends up a heroine as a result of the story.
My understanding of とした is also a little fuzzy.
Help is appreciated!