Not a damn thing

You couldn’t do anything but stare at the sight in front of you, watching your girlfriend slobber her mouth over the cock that she had been pining over for over a week now, finally able to taste it for herself regardless of who it ended up being attached to. Of course, what could you do anyway? This wasn’t going to stop, not after seeing Allison’s face moving in on her own to give into temptation and have a taste of your own sister’s snake-like cock flopped out in front of her.

Even now as your sister was bucking her hips and sinking what she could into your girlfriend’s through she made no struggle against any of it, just lewd gags and suckling noises as Allison drooled on herself down her chin.

As Fiona saw Allison’s eyes closing she gave a harsh yank on her hair, causing them to reopen and flick back up to look toward your sister. “Eyes up here, sweetie, can’t have you pretending this is anyone else~”

You couldn’t really tell what Fiona’s goal in all this was, it seemed to be all about putting you in your place, but she seemed more into things than she ever was before. Your sister’s eyes were intensely locked on your girlfriend’s, feeling in the moment as she fucked the first few inches of her shaft into the back of the girl’s throat, though the angle she was holding Allison’s head at was preventing her from going any further than that.

Each of Fiona’s thrusts forward was now pushing Allison’s head backward, slowly releasing her hair until she was laying fully back on the bed. You sister leaning her body forward at a much better angle, allowing her to sink much further into your girlfriend’s willing throat. Fiona letting out a much contented groan, snapping her head back and closing her eyes. She was now fully looming over the smaller girl beneath her, her pelvis moving closer and closer to those soft supple lips with each and every downward thrust.

The lewd wet ‘gluck’ing and gagging noises echo through your ears from the doorway. As your eyes looked on at every detail, you swore you could see Allison’s tongue flicking about beneath the weight of your sister’s thrusts. Watching your girlfriend letting your sister use her throat as a fuck sleeve, this wasn’t just kinky fun between lovers any longer, you weren’t even in the equation.

Fiona’s eyes flutter open in a moment of bliss, her pelvis finally pressing down to Allison’s soft lips, and your sister’s eyes once again meet your own while sporting the cheekiest of grins. You could even now hear the faint sound of your sister’s balls slapping down against Allison’s chin, a little cherry on top to the symphony of sounds coming from the two on the bed.

Fiona let a little moan escape from her lips, the tension in the room rising, you knew what was about to happen. Your sister grips the top of Allison’s hair and slammed her hips downward one last time. You could even see your sister’s sizeable balls tightening up and your girlfriend’s eyes shoot open, she was pumping her warm fresh seed down her throat. Fiona’s eyes were still locked with yours, making sure you knew where your place was, there wasn’t a damn thing you could do about it.

After a few more agonizingly long moments Fiona held the girl’s head down and slowly slid the shaft from her throat and mouth, leaving her fully erect shaft dripping and shining with a mix of thick saliva and fresh cum. Letting the last few spurts of her cock dribble directly onto your girlfriend’s face as if marking her territory.

Your sister smiled down at her as she slapped her massive cock against Allison’s face, the wet slaps accompanying the panting of your girlfriend as she tried to catch her breath, a string of saliva and cum connecting from tip to lips for a brief moment.

“Mmmm, that was fun, I’ll be sure to make this a regular occurance” Fiona smirked as she talked down to Allison, slowly stroking her messy cock with one hand while the other was still making a mess of your girlfriend’s hair, holding it in her fist.

(Apologies for taking a bit on this one, life got busy for me this week. For this next part I really was caught between two choices. To continue with this encounter to its 'obvious conclusion', or to have them return to dinner for now as to not rouse too much suspicion and drag things out a little till later a new scene where that conclusion happens, perhaps a little more teasing and mischief in the middle. So whichever is the more popular option in the comments is, I'll go with that, and I'll try to have the next post out MUCH faster :3)