It's time to finish this.

John managed to pull away from kissing his little sister to look her in the eye as his orgasm approached. He licked his lips, tasting her saliva on them, and looked down at her gorgeous little body, savouring every moment of the act they'd performed before it was soon to be over.

'Are you going to cum, John?' Louise said, not bothering to quiet her voice for the doctor anymore. 'Going to cum into your little sister's hand?'

'Louise,' the doctor suddenly spoke, though John was far past the point of caring. Louise didn't even slow her pace as the doctor continued. 'Try to catch some of his semen in this, for testing.'

Louise took a small vial with her free hand, thankful that the doctor hadn't heard her admit their relationship, or had simply ignored it.

With the vial ready, she pumped faster and faster while keeping her eyes on her big brother, watching him squirm as his orgasm approached under her hand.

'That's it,' she said, her voice soothing and husky as he began to twitch. 'Cum for me, John.' She moved in to kiss him again, a quick probe with her tongue, before leaning into his ear again, and whispered, 'cum for your little sister.'

The words sent shockwaves of pleasure through him, and his balls tightened as his juices pushed their way through his shaft and exploded from his head with more **** than he could ever remember. His load shot all over Louise, each strand leaving a trail on her clothes, before she quickly moved her vial to catch some of the hot, sticky liquid.

She kept her hand slowly moving up and down his manhood, squeezing every last drop of the stuff from her big brother, unable to suppress her grin at the fact that she, herself, had gotten him to this point.

John opened his eyes as the last few waves of orgasm shot through him and out of his head, looking at Louise as he slowly faded back into reality. And the reality, he realised with some unease, was that his little sister had just jerked him off to completion. He looked at her lips, where he'd just been passionately kissing her, then down where his ropes of cum had stuck to her shirt, then further down where her hand was still gently holding his hard dick, and her other hand held a vial full of his semen.

'Well,' Doctor Harper spoke, further reminding him of where they were, 'that was interesting. A little strange at the end there, with the whole 'little sister' thing, but I don't judge. Every couple has their kinks.' Doctor Harper laughed, the two siblings trying to join in, though both of their cheeks flushed. 'How does it feel now, John?'