Then her body collapsed on top of my own, moving up and down like she'd just run a 100 yard dash, her lungs seeking oxygen.
I managed to pull my self out from beneath her and got off of the bed.
Without saying a word I retreated to the bathroom and returned with a tube of KY jelly. I unscrewed the top and dipped a couple of fingers inside. I spread my sister's ass cheeks and began fingering her asshole with the KY.
Donna figured out what I was doing fairly quickly and raised herself up onto all fours, spreading her legs apart widely in a lewd manner. She reached under herself and began playing with her pussy again as I continued to ready her asshole. Some more KY was liberally applied to my own steel-hard erection, in preparation for sodomizing my sister.
My cock could wait no longer for its satisfaction, so I got back onto my bed and positioned myself behind my sister. I grabbed my cock and placed its engorged head at the entrance to my sister's now well-greased asshole.
Even with all the lubrication, entry into my sister's anus was difficult. But I continued in my efforts, as my sister provided assistance by pushing back her ass to welcome my cock meat inside her.
Once the head of my cock was inside Donna's asshole, things seemed easier. I was enthralled at the sensation of extreme tightness on my cock that my sister's asshole provided. Deeper and deeper, bit by bit, my prick penetrated inside her bottom. My balls were positively on fire at this point. My cock felt as large as a telephone pole!
This was simply too good to be true.
I heard the sound of an animal grunting and then realized that it was myself.
I muttered things like "fucking your ass" as my strokes continued, now faster and deeper than before.
Donna thrashed about underneath my assault, crying in a combination of pain and pleasure, saying "Harder!" and "So good!"
It was not long before my tortured cock sought relief and I could feel a flood of hot cum erupting from my balls and racing to exit.
I pulled my cock from my sister's asshole and unleashed a savage amount of hot cum upon her ass cheeks and lower back. It splashed against her skin and collected in pearly white pools. I moaned unintelligible noises as the jets of thick, white cum shot out of the tip of my reddish-purplish cock head like lightning bolts.
With my hands, I spread the cum across my sister's ass, her back, the back of her legs. It was truly an astonishing volume of semen that I had emitted onto her body, and I wished she could have seen it.
My fingers were soaked with a combination of my sister's pussy juices, KY jelly and my own spunk as I fell onto my back, too exhausted to go clean up right away.
My sister rolled over and lay beside me, falling to sleep with her head on my chest and her right hand cupping my balls, purring contently in the afterglow of our forbidden incestuous acts.
I, too, fell asleep, waking perhaps an hour later to find Donna gone from my bed. I showered, dressed and went downstairs to find Donna watching TV, herself looking all cleaned up and wearing fresh clothes.
"Hi Brian," she said nonchalantly, not lifting her eyes from the TV. "Mom and Dad just called. They just left the restaurant and will be home in a few minutes."
"How was your nap?" she grinned.
"Donna, it was the best," I replied with a devilish smile.