Both of them give in completely

Sue didn’t even notice that she was being stripped of her clothing, all she wanted to feel was the warmth of her sister’s mouth. Only as she felt the fingers of her sister at her clit she stopped kissing Jess and eagerly stripped her too. In a few seconds, both girls laid naked beside each other at an empty train stop, fingered themselves, kissing passionately and moaning their lust in the air.

Sue didn’t even notice Rebecca, who was watching them with interest and touching herself. The blonde girl stood beside the two girls, who were way too busy kissing and touching each other. Sue's mind has lost all rationality, all she wanted was to fuck her sister until they both came.

It happened after a minute of intensive fingering. Jess gasped loud and closed her eyes, and Sue could feel her sister's inner walls clench, then Jess let out a loud moan.

“Oh, Fuck!” Jess moaned. “Oh fuck…I can feel it! It’s so good!”

Sue didn’t stop her work at her sister’s clit and after a few seconds, Jess cried out, “OH SHIT!”

Sue watched in awe as her sister’s body changed itself before her eyes. Her small breasts grew into perky tits, her stomach got more muscular and her hips were widening. But the greatest change happened at her head and her back. Little grey horns pushed out of her dark hair and crooked backwards, and at her spine, black batwings grew out from her little body.

“Oh my god,” Jess mumbled and touched her new horns, while her wings grew even wider. “It’s marvelous.”

“I want it too,” Sue said, thrilled by the thought of it, “I want to be as sexy as you.”

“Oh, you can bank on that,” Jess said grinning.

She grabbed her sister by the neck and pushed her onto the bench of the train stop, than she kneeled before her and started to lick Sue’s invitingly wet pussy. With the first lick, Sue moaned even louder than her sister. It was better than anything she had ever felt. Jess licked her clit while she used two of her fingers for the proper stimulation as well. With her other hand, Jess grabbed her own pussy and stroked it eagerly, while Sue grabbed her breasts and pinched her nipples.

“Oh my god,” she moaned, “You are so good sis, don’t stop, please!”

Sue could feel the changes in her body, even though she was too aroused to look at them. Her breasts grew until she couldn’t grab them with her whole hand, two horns, not as big as her sisters, pushed out from her hair. She looked at herself and saw that her whole skin colour had become purple, but the biggest change was yet to come. Jess licked and fingered her full of lust and Sue could feel her orgasm coming, like an enormous wave, which washed every doubt out of her brain. Another voice was speaking in her head now, a dominant, male voice, “INFECT, SPREAD, FUCK!”

Sue instantly knew that this was her new master and was fully aware of what happened to her. She was infected, but it wasn't a disease. It was a gift.

“OH MY GOD, YEEEES!” Sue yelled

She orgasmed hard in her sister’s face, and as she hit the peak of her climax, Sue could feel it. Down from her spine, right above her new fleshy ass, a new body part emerged: a long, fleshy tail, just as purple as Sue. She instantly orgasmed again and the new tail grew and grew, until it was long as her leg. She could feel the new nerves and knew it would work for her just like a dildo. She stood up and looked down at her new body.

Her new tail seemed to have its own mind. It stroked her skin gently and the tip moved to her pussy. Sue wanted to feel it inside her and she knew she could navigate it into her. As the tip of her new member touched her wet entrance, she gasped. Her sisters tongue felt great, but the feeling of her new tail already made her cum again.

“You look so gorgeous, Sue," Jess said excitedly. “Oh my god, I want this tail in my pussy, it must feel so good.”

Sue grabbed her tail and licked at the tip of it. It tasted divine and Sue felt that she was lactating a bit by the taste of her new limb.

“Oh my god!” Jess exclaimed now with a lusty moan. “Please put that thing inside me!”

Before Sue could approve this suggestion, a voice made her and Jess look up.

“Care if I join you?” Rebecca stood before them and grinned with satisfaction.