Sadly I knew someone whose relative was in a position such as this.
She was forced by her "boyfriend" to prostitute herself and he took all the money. He used "love" as the weapon to keep her and beat the shit out of her, when she questioned him, wanted to stop or leave him.
He threatened to kill her family members if she ever left or if anyone found out about her/him.
Since he kept all the money, she had no means of leaving.