Why not both?

"I wish Sarah forgets what Natalie said and views her as her twin sister, who she isn't threatened by and she's happy and excited to have."

Natalie smiled. "Good wish," she said, and Tom could feel a few memories start to shift. Natalie pulled out her phone and opened to a message thread between her and Sarah — it was filled with hearts and encouragement. "Guess it worked," she said, and slid onto Tom's lap. "I suppose I should thank you for giving me such a loving sister, shouldn't I... daddy?"

Tom looked a little bewildered. Was this okay? He was still married to Sarah, after all. Natalie saw his hesitation and looked a little hurt. "Please, even if Sarah doesn't know, you know we're the same person... You told me I'm as much a part of your life as she is. Remember?" Tom thought about it for a moment, and realized she was right. He'd created Sarah's double; she was, effectively, Sarah. And it didn't help that she felt so good on his lap. As he ran his hands down her waist, she smiled, and kissed him.

"How should I thank you daddy?" she asked, unzipping his pants as she moved on top of him. Her fingers ran down to his hard dick. "Do you like this? Do you like when your little girl touches you here?" He pulled her in closer and kissed her. After their lips parted, she looked at him with an unfamiliar expression on her face.

"You know," Natalie said, "Since you just created me... Even if I have memories of doing things with you before..." she slid to the ground beneath where Tom was sitting on the couch, and slowly pulled his pants down. "This is the first time," she continued, leaning forward, "That I've actually had your dick in my mouth." She took him gently inside her lips and began licking and bobbing, never breaking eye contact. After a few moments she pulled off and slid her hand up and down his spit-lubricated shaft. "The real thing tastes so good," she said, and Tom leaned down to kiss her. "You're doing so good," he said, and she kissed the tip of his cock.

"It would be so hot if you could actually change me with wishes," she said, kissing him again.

"What would you wish for?" Tom asked.

Natalie paused and seemed to consider. "Well, to be honest... Since I'm in student government, the first thing I'd probably do is ask you to make me smarter," she said, laughing. "I guess I'm a bit of a nerd."

Tom laughed and Natalie went back to sucking him. He ruffled her hair and said: "I wish Natalie was smarter."

Natalie popped her head off his dick and rolled her eyes. "You know that doesn't work, dummy." Then she paused to think. "But, actually... I wonder if we could do it through wishes on Sarah..." She slipped her shirt off and began rubbing his dick with her breasts. "Wish that Sarah's twin had bigger boobs."

Tom's eyes widened. It was worth a shot? He did, and...