From what I remember it's just that, as the princess, the original villain felt like she was being robbed of her belongings (ML) and the spotlight was turning to the FL, who was now at the center of the social world.
I cant realy blame the villainess tho.. living her life being compared to her brother.. only to then have her finane bring in a girl he becomes infatuated with and then for the "Saint" to call her a fake princess just because she isnt a saint.. when the saint herself is a fake saint.. that would make anyone bad especialy if they are young.. and to top it all off... the girl her fiance cheats with her one turnes out to be her cousin and the saint and gets worshiped as the "real" princess just because of this.. That has to hurt.. to be told that dosnt mater if your father was the king and yoru mother was the queen.. your twin brother is a prince but you yourself are called a fake.
At chp 1+ is it just me or I'm just confused as hell, the original Villain hated the ml, but harassed the fl? Why?