My Step-Sister Wrecks The Car and is in Big Trouble

Upon returning from the job, I was surprised by the sight of my step-sister's new car damaged in the driveway. Our folks are sure to go ballistic when they learn about it. In a desperate plea for assistance, she turns to me, aware that I have a contact who could possibly repair her vehicle.
Pressing upon her that it won't be simple, I make it clear that she'll need to do me a favor. Admiring my step-sister's appeal for quite some time, the opportunity to have sex with her now presents itself. If she desires my aid, she must allow me to have sex with her, though I remain unsure if she can handle the full length of my manhood.
Discovering rapidly that she can, I position her on all fours and vigorously thrust from behind before ejaculating inside her and instructing her to tidy up.