Finally, both girls came, and Alison rolled onto her back, exhausted. I gave her back her pussy, and Lina cuddled up with my girlfriend.
"Ok girls, rest up, because now it's my turn."
Lina gasped. "No way, you're my sister!"
"Please. You're so hot for me, you're getting wet just thinking about having sex with me...in fact, you're going to cum again right now!"
Her eyes widened, and she suddendly pressed her hands to her crotch, trying to keep in a flow of liquid from her urethra. It oozed out between her fingers, and soaked into my sheet. But I didn't care, the loss of control was total, and it made me feel...good.
"Alison, be a dear and clean up my sister, she can't seem to stop."
Alison got up, a bit groggy, and pushed Lina's hands away, pressing her face into my sister's crotch and lapping up the other girl's juices.
By this time, even more warped desires came to mind. "Lina and Alison, your bodies are now going to merge together. This will feel amazing, like the most intense sex you could ever have. Alison, you will get even smaller and younger, becoming a nine year old girl. Lina, your body will get larger and stronger. Your head will vanish, and like a centaur, Alison's hips and abdomen will come out of where your head used to be. You'll have to crawl around, but you'll easily support her weight. Your brain will move into your womb, and be perfectly safe there; you will both share senses. Lina, you will be totally subservient to Alison, and respond to her will."
The two girls' flesh melded together like putty, and their voices keened in unison as they both experienced an intense orgasm...which soon became a shared orgasm as they merged. Alison's breasts vanished as she became a pre-teen girl, her legs melding into Lina's upper torso. I could see extra muscle being added to Lina's arms.
Lina's head sank into her body, and within minutes, the transformation was complete. Lina rolled onto her hands and knees, and rose, so that Alison could be upright.
"Oh wow...this is amazing. I can feel her, in me." Alison's voice was high-pitched and young; she was even more beautiful at this age than ever.
"You look amazing..."
Alison turned and preened at her reflection in the mirror. "Do I? Yes...I do."
"Make love to me."
She smiled and I lay down on the bed. Lina lowered herself so that Alison could lean forward, her little tongue darting in and out as it tasted my skin.
"I want to orgasm like Lina did...I want to explode with girl-cum...I want to have multiple orgasms at the same time...and I want you to make that happen."
"Of course, my beloved."
And she did.