Blackmail - manipulate (someone) into agreeing to do something by means of threats or coercion for another person's gain or sexual pleasure. Step-sis's secret is good with you... right? Wrong! This is the exact leverage you needed to convince her to finally spread her legs and do everything you ask of her! She is now your very own personal play-toy to explore and enjoy! Best thing is, since you have something over her... no one will ever find out!! Now come on sis. Don't just spread your pussy lips, act like you enjoy spreading them... lol.
