Responses
I can't agree with that since she's the one (unknowingly) encouraging the FL to go after the Villainess. She's literally the one giving her the advice that push her to confess to Rosie.
She doesn't feel like a plot device but rather a careless character that get too comfortable talking with her brother.
Let me guess... the sole reason why the sister was added out of nowhere was because the author needed to somehow bring up the possessing to the male leads??