I have been interested in the same question OP. I looked into her background to see what could have motivated her, here's what I wondered...
She was one of nine kids but was the youngest and the favourite child of her dad, apparently they were very close.
He was a very wealthy media mogul and she grew up in what one news outlet called "unimaginable luxury."
He committed suicide in 1991(either that or he fell off his boat and drowned, but suicide is suspected.) It is speculated he possibly committee suicide because he was engaging in pension fraud to fund his own lifestyle, basically dipping into employees' pension pots and had spent $20 million of it.
Upon his death, Ghislaine was devastated and also she went from having unlimited cash to having to exist on trust fund cash of $100,000 per year.
A friend said she 'worshipped domineering, wealthy men' (perhaps men like her dad). Perhaps she was trying to find a father replacement. Jeffrey Epstein was so wealthy he was able to give her the lifestyle she wanted, and in return she gave him society connections, apparently he was socially awkward and shy.
They had a 3 year long relationship and she wanted him to marry her but he refused. Their relationship turned into something more like a friendship eventually.
I wonder if the trauma of her father's death and the loss of access to the old lifestyle she led, had her intent on finding someone to replace him, and she thought Jeffrey was that person, and she was willing to hang on to him no matter what, because he was able to keep her in the lifestyle she had been accustomed to, when she was younger.
This is me playing armchair psychologist.
I think she was traumatised but I also think she's a vile person. Apparently she absolutely treated the abused young women like something she found on her shoe. Maybe through jealousy that they had something Epstein wanted (yuck) but she held out hope for a long time that he would marry her. So perhaps that's why she gave him what he wanted.
So in conclusion, I believe she was motivated by money and by trauma over the loss of her father and resources when she was younger, and would do anything to hang on to Jeffrey Epstein. Maybe she's a sadist, too. That's what Virginia Guiffre said, she behaved sadistically towards the young women they ensnared, the maximum pain she could inflict.
Bear in mind some of this info about her background is reported in places like the Daily Mail so not sure how reliable it is.