Kat leaned in, she somehow had a pail in her hand.
"What's that..." Dan started, then stopped as realization stole over him. There was only one thing a pail could mean. "Wait, top! You - you can't milk me!"
Kat just grinned "Wait and see." She reached over and pulled at his lower teats. The feeling was overwhelming, like an orgasm but somehow better. The milk started flowing out easily.
Dan wanted to get away, wanted the unholy pleasure of being miled to stop.. but he sat there as Kat slowly filled up the bucket with his milk.
"Why...why can't I get away?" he looked at his sister, large brown eyes full of worry.
"You're changing more dear brother. Haven't you wondered why you haven't been more shocked? More outraged? I would think a fine young man would be outraged at becoming a cow."
Dan stared at her a few minutes... it was a good question, but one he couldn't seem to get an answer to... then... "My mind... it's changing too isn't it?"
"Very good." Kat picked up the bucket and smiled at him "Your mental changes will be slower, but eventually you will come to think of yourself only as what you are now - a female cow. Your memories of being a male human will fade away. I'll be back in an hour to check your progress." she laughed and vanished with the milk.
Dan stared at where she had been. He needed to get up, to get help... but as he moved he couldn't help but feel around his udders. It was like having six dicks just for him to play with... he'd get up in another minute...
***
Kat returned to find Dan playing with his udders. She smiled, things were going perfectly. She made a small noise and Dan looked up.
"Hi Kat." Dan greeted her mentally. Physically he could only moo. Her appearance meant that an hour had gone by. That also meant Dan had changed a lot mentally...