Sin is a statistic in Princess Maker 2 that indicates how much unrepented evil a person has done. It can go up to 999.
Sin increases mainly by Olive killing animals or monsters or entertaining people in the sleazy bar and cabaret. It is increased by 25 points by accepting Lucifon's gift and 80 points by accepting his liquor. Olive will only accept the liquor if her faith is below 20. Her sin will also increase if she attacks loyal citizens, but doing that carries risk of arrest.
Sin can be decreased by either donating money to or working in the church.
Sin is the main factor that determines the tone of her letter to you at the end of the game. If her sin is below 100, then she is grateful and refers to her employers respectfully. If it is above 100, but below 500, then her tone is uneasy and dissatisfied and she admits that she is not grateful and she refers to her employers by their job titles only. If her sin is 500 or more, then her tone is sarcastic and she treats you to her unmitigated derision.
A sin of 500 is one of the three requirements for her to become the ruling queen of darkness. The other two requirements are combat reputation 300 and magical reputation 200.