Dan's mother explained to her the basics of being a witch, gave her a book of spells to read, and then suggested that Dan could change all of her male clothing into female clothing or summon an assistant for practice.
Dan considered magically changing her clothes. It was something that she was going to have to do eventually. But decided it could wait. She was sure that her decision to remain female was the right one. For one thing she got to be a witch, for another thing being a female just felt right (especially being touched in a certain area, she was eager to learn what it felt like when the person doing it wasn't her evil sister.) The fact that neither of her parents had any objections to her being female simply reinforced the decision. Even so, she wasn't sure she was quite ready to go through as visceral a reminder that she was recently a he as all of her clothing.
Summoning an assistant seemed interesting though. She read the appropriate things, said a few words, waved the wand a bit, and suddenly in her room appeared what looked like a girl about her age. The girl was attractive, she wore a white skirt and a pink tank top, she was he same hight as Dan, with a slender frame and what Dan guessed where C-cup breasts. Her hair was red and wavy.
"Um, hi," Dan said.
"Hello, how may I serve you?" she asked.
"Information, I guess. The book told me how to summon you, it didn't actually say what you are."
"I am a servant. I exist in for as long as you need me, in whatever form is pleasing to you," she said simply.
"So what were you before I summoned you, and what happens when I no longer need you?"
"I was pure energy, and will return to energy."
"What's that like?"
"It isn't like anything, before you summoned me I had no mind, no personality, no awareness. I will return to that when you are done with me. Until then I will do whatever you ask of me."
"So you're a slave and then you die? Does that bother you?"
"No. It is what I am. If I were not formed to obey you then I would not exist at all. Following orders is my purpose. You can order me to be free, if it pleases you."
"Do you want me to?" Dan asked.
"No. What would I do? I'd be all alone. I don't want that."
Dan thought for a moment, "I don't want you to go away. I want you to remain my assistant for as long as I should need or want one. Stay even when I do not have a task for you. Also, I have one order for you to follow above all others: Don't be evil. But I'm not sure what to have you do."
The girl smiled and leaned forward so that Dan could see her breasts through the neck of her tank top. "I am well versed in many things, not just magic. If it pleases you I could introduce you to the pleasures of the flesh."
Dan's mind flashed to earlier that day, the intense pleasure she had experienced. "Tell me, do you like cows?" The girl considered then gestured in the affirmative, and Dan removed her clothes. She grew the udder first, flesh expanding outward, then teats extending from the sack that formed. Next a tail popped out. Her ears changed next, she pointed one at the magical girl when she heard a giggle, finally the time had come for the big part. Dan got on all fours as her hands and feet became hooves, a coat of white fur sprouted, with the customary black spots. Muscles moved, bones readjusted, weight was added, and soon Dan was a cow.
The final touch was when Dan felt that her udder was full of milk. Dan communicated with the magic girl via telepathy, "Milk me, and show me those 'pleasures of the flesh.'"
The girl summoned a stool and a pail. She sat beside you and milked you, not in a rushed way as your sister had done, but slowly, sensually. She gripped a teat softly, squeezed gently, pulling downward slowly. It sent shivers of pleasure through Dan's entire body. She didn't speed up or slow down, instead sending Dan to a plateau of pleasure unlike anything she had ever known.
The pleasure subsided briefly, but before she could ask what happened she felt the magic girl's mouth close around one of her teats. As the girl began suckling Dan found herself in heaven. Her sister's curse said that 'the animal deep inside' her was a cow. If this was what being a cow was like then that seemed just about right.
By the time Dan's udder was dry her vagina was soaked. The magic girl moved to it the moment she had sucked the last drop of milk. She placed her had inside and just ... rubbed. Multiple orgasms seemed a gift from god, Dan was never going to be male again. She just hoped that human orgasms felt this good.
The magic girl kept the pleasure going until Dan was on the verge of collapse. Dan turned herself back into a human and then she did collapse. The girl helped Dan onto her bed, and then climbed in with her.
"That was incredible," Dan told her.
"I rather enjoyed it too."
"Is that because you have to?"
"No. I only have to do what you order me to. If you ordered me to enjoy something I would try my hardest to do so, but I might not be able to. I enjoyed it because ... well that's what I enjoy. So many of my kind are employed in nefarious activities. I have encyclopedic knowledge of thousands of years worth of plots to take over the world that my kind have been forced to participate in. I also have encyclopedic knowledge of every sexual act possible. Within a second of my creation I decided that I would much rather use the second set of knowledge. It seems a much more noble pursuit."
"That's ... " Dan's eyes closed, "nice," she fell asleep.
The magical girl stoked Dan's hair, "Beautiful master, I have decided that I shall be known as Cinsi. It is Kurdish for sex." She lay back and placed her arms around Dan. Dan had ordered her to remain, but not said what to do. She decided to sleep.