Suck her off.

The excitement building in you is too much, and you slide off the couch onto your knees, practically begging for her to stuff her cock in your mouth. She smiles and plays with your hair a moment - then grabs your head and shoves it forward onto her equally aroused package. You eagerly fit as much as you can of her eight-inch shaft in your mouth and start rhythmically moving your lips back and forth, back and forth.

All the while, your sister continues emitting cries of animalistic lust, and starts practically forcing your mouth onto her dick. Even though you've had no practice in the field, you seem to have a talent for the art of oral, given the ecstatic moans she repays your deep-throating with.

Finally, neither of you can take it any longer, and as you climax, spraying yourself all over the couch, she starts pumping your throat full of her own cum. Finally, she collapses back onto the couch, cock still in your mouth, as you carefully suck every drop off.

Of course, both of you are still plenty excited, and as she smiles mischievously, you wonder what she'll do with that incredibly hot member of hers next...