I Help You... You Help Me?

I may not be the most muscular or intelligent, but I can recognize a promising chance when it presents itself. My stepsister, Lily Starfire, has a voluptuous figure with ample breasts and a plump, desirable buttocks. When I mentioned needing guidance in matters of intimacy, she initially dismissed it with a chuckle.
However, when she found herself in need of assistance with her math studies and proposed a mutual agreement, things took a different turn. She would educate me in certain activities, and I would in turn help her with math. An opportunity, indeed.
As she uncovered her sizable breasts, not only did my interest pique but so did another part of me. Stimulating my erection, she became aroused herself. Straddling me, she guided me inside her wet vagina, and as she rode me, I fondled her breasts.
Taking her vigorously from behind, her generous buttocks quivered as she climaxed. Despite her initial displeasure when I ejaculated inside her, I am dedicated to providing her with diligent tutoring in the upcoming days.