........
spoilers below:
proceed at your own risk....
btw what I say might be a little inaccurate (details) since this is from awhile ago...
you are mostly correct:
the little sister didn't originally have powers, she tried to steal MC (her older sister's powers) and then do away with her. OG MC (before possession) was practicing/recovering her little bit of powers that weren't stolen by making flowers bloom (this is why sister & dad were upset at MC's flower arranging).
little sister, father, and prince are working together
So the three of them know about the power being stolen?
Is the sister evil or something???? Is she a minor antagonist here??? Also that prince is so suspicious