Why not!

John was quite enjoying his new toy, it was quite amazing to have the power to turn the woman who gave birth to you into a tight holster for your cock. it was a good start but it didn't stop him from continuing. He and Erica had always been close, and the mutual love of stuff like this had always been boiling just below the surface. Erica probably wouldn't have joined him so eagerly if she had watched the whole process on the breakfast table. John had come down the stairs and within a moment her parents had been joyfully making out on top of the table Erica had watched in addicted fascination as her father had become smaller and less complicated his entire body compacting arms and legs withering away his hips and rear becoming bulbous before softening out into a fake phallus.

After a few minutes the ravishing redhead watched her mother remove the lubricated dildo that had been her father from her lips.

Yet before she had been able to do much of anything her own body had transformed. Erica had watched transfixed as her brother had snapped his fingers before happily burying his sizable erection into their mother's twat.

Erica had watched with amazement as John had committed **** in the front of her. Showing off his power his eyes daring her to emulate him.

She watched her mother slowly become less human, arms and legs disappearing into her semi translucid body. Transforming her from a vibrant woman into little more than an elaborately detailed hole for pleasure.

Erica had finally given in at that point grasping the toy that had been her father and feeding it into her hungry snatch.

She'd watched as her brother continued to squeeze, her mother shrinking until she was literally just large enough to contain her son's impressive erection.

Then John snapped his fingers again it was time to see exactly what Erica was going to become. She felt tears forming in her eyes, a mix of fear and arousal as her fingers began to change around her father's shaft. Her fingers were already changing becoming harder and more artificial spreading and making her more like an inert object with every moment . He was really going to do it, fulfil her deepest darkest most depraved desires. She was certain she was going to love it.

(feel free to make suggestions I'm kind of writers blocked)