What Should I Do ?

Lastnight my step sister told me I could eat her pussy tonight if I let this guy she knows fuck me while I do it . I've been trying to get her to let me eat her pussy for about a year and at first I was totally against the offer . But I guess I'm going to let him fuck me so I can taste her finally. I really hope I'm on my back with my legs up with her on my face with him jerking me off while im getting fucked but idk if he will or not . If hes not jacking me off while hes fucki g me how can I encourage it without saying anything. I dont want to jerk myself