Ambers mind said no, but unfortunately for her, her mind had very little input in this situation. Right now her penis was in charge, springing to life at the sight of her little sisters transformed state. Gemma breathed heavily "Please.. Put it in...". Her darkening pussy was now dripping with lubricating juices having gone under considerable changes to allow her to fit Amber's erect penis. After what felt like an eternity Amber succumbed to her instincts and was more than happy to oblige her wanting sister. Both girls shivered as Amber slowly inserted the tip into her pussy as inch by inch she slid into her.
"Ohghh" Grunted Gemma. "Nhhh! It ugh, feels so good! It's filling me up!" Amber had finally completed inserting her mammoth penis into her sister, her pelvis was now rooted firmly against Gemma's ass. Gaining confidence she begins to speed up her thrusts, with each blow accelerating her own transformation, already in its final stages. Her hands which once rested firmly on her sister's buttocks not longer had the dexterity to hold on and she found herself slipping off and taking on a more feral position. Amber's body began to rapidly gain mass, her torso swelled in size enveloping her large breasts as it did, shredding any remnants of her clothes. Her legs locked into place forming a right angle with her body. Fur began to lengthen forming a solid brown coat all over her body. All that remained of her human form was her face, attached to a thick donkey neck that protruded from her body.
Gemma on the other hand was lost in throws of ecstasy. She couldn't believe her luck, this was surely the best day of her life. She too could feel the changes spreading across her body as her sister pounded her pussy again and again, only she relished the changes, welcoming them in, offering no resistance what so ever. She could feel the light tufts of fur sprout across her body as well as her ass becoming larger to accommodate her changing genitalia. Although she did notice something odd. Her sister's genitalia had changed completely from a vagina to a large thick penis which was now in the process of fucking her silly, however she had kept her's. It did change, swelling in size to accommodate the length of her new species as well as darkening in color and re positioning itself. But she felt another sensation by her clit. She could almost feel it growing...
Amber continued to thrust until she reached her limit. Letting Gemma know full well that she was about to blow her load deep in her sister's womb. "Ambheeer!" She half cried, half brayed. "I'm cuuuuuming! It's cumming! In you pussy! Heee haaaw! Heeee Haaaaw!" And with a final mighty bray she let loose and sperm shot from her penis completely filling young Amber's body. As sperm flooding her womb for the first time Amber too came, letting out her own triumphant bray. Tired and sweaty she slowly pulled herself off of Ambers cock as inch by inch it slipped out of her.
She looked down at her sister, who still surprisingly had some humanity left in her, in the form of her face, however she could see this was not going to last much longer. "Gemma?" Amber said. "You still in there?"
"Yes. But not for long" She replied her English mixed with donkey like sounds. She continued "That was the most pleasurable thing i've experienced in my entire life. Right now I both love you and hate you" She giggled to herself. "Just promise you'll change me back." And with that Amber's humanity slipped through her fingers as the final stage of her transformation took over, completing her transition into a donkey.
"I promise" Gemma replied. "Although, i't can wait a while, I'm not quite done with my changes. Yet." And with that she went back to the mirror to once again look over herself. When she got there she was shocked to find...