After getting on her knees so that Damon can pound her from behind, Riley orders him to lay on the bed so she can climb on top of him.
Moments later he pulls out one last time, and covers her pussy with his jizz just as she demanded.
When Riley Reed walks in and strips naked, Damon hides in the closet and strokes his stiffie.
After seating herself on his stiffie, Riley takes turns grinding her hips and letting her stepbrother do all the work as she takes what she wants until she squirts everywhere.
Only after her stepbrother has jazzed in her mouth does she realize that she has mistaken his identity.