"Only touch him"

"Make sure you're just touching him." Greenie says as she bounces on you. "I'm still your sister you know."

"Oh I remember." Katie responds, staring up at her sister's naked body.

This seems like a completely reasonable request, you think. After all, despite how indescribably naughty this whole night has become, the two of them getting physical would cross a line that you're not sure can be uncrossed.

As you think all this you watch as your girlfriend pulls her shirt up over her head, then slowly, methodically undoes her bra. Now Greenie can see her sisters tits just as well as Katie can. With the loss of your pants and the loss of Katie's top, there's definitely a sense that the evening is continuing to escalate. That penetrating Tess was just the beginning. You look up at Tess's nude back. You miss glancing at her bouncing breasts. But she has a nice back too.

And suddenly you have the sensation of Katie's tongue on your balls. You glance down as best you can and what you see confirms what you feel.

It's amazing, Greenie's twat squeezing your cock and Katie's tongue swirling over the rigid skin of your scrotum as if she's never tasted anything quite so good in her entire life.

And its shocking, considering how close that places her mouth to her sister's pussy. At one point Katie's tongue works its way up to the bottom of your shaft just as Tess's twat sides down it, and you swear they must be making contact. Tess even lets out a little moan. But you have to assume it wasn't related.

Then you swear it happens again.

Then again.

Tess pulls her arm from your grasp and she runs it through her hair in a moment of what must be high anticipation, arousal and exhaustion and stares down at her sister. "Okay just do it."

Katie pulls her face away from your crotch and glances up. "Tess?" as if to ask if she's sure.

Tess is trembling now. She knows what she's about to say can't be taken back and what she's about to ask to be done can't be undone. "Fuck what I said before." She insists. "Just do it. Do it. Do it already."

And Katie responds by placing her tongue at the bottom of your balls and licking; maybe the longest lick in the history of humanity. Up your scrotum, up your shaft, straight through the point where it plunges into her sister's pussy, up Tess's clit, up her sweaty body, past her waist, through the middle of her breasts, up her long neck and depositing her tongue into her sister's willing mouth. And they kiss, long, passionate, unrestrained.

You just watch, silently, knowing you're witnessing something completely beyond anything any of the three of you had ever expected to see or do in your life. But you're filled with this sense and you hope that Katie and Greenie are too, that it's okay; that in this moment, it's right.

The two pull their lips apart and you barely hear Tess whisper. "I love you, sis."

And you can't help yourself any more. Your cock is going to explode