Short answer: she's completely delulu but her goal is to cement herself in the highest position possible.
Long answer: she's a narcissist and classist. She thinks she deserves it for suffering the family she was born into and that she's destined for greatness. She sees the MC as someone undeserving and someone who squanders her position in life, aka she should have been the stereotypical evil rich girl she emulates behind the mask instead of looking like a happy airhead. By seducing the men, she feels she's going up the ladder and she'll eventually get what she deserves, which seems to be everyone's attention and perhaps she can even drop her facade. She sees men as tools and pretty women as obstacles she has to "cunningly" eliminate.
Why does the white haired girl feel so entitled to the female leads life? She not even related to the family or adopted. Couldn't she just use the energy instead to go marry a noble or something? Seems easier imo