Jenny thought for a moment before stating her answer. "Hmm, I think I want to be your sister."
"What?" Jeff reacted. "Why would you want that?"
"It seems fun. If I'm your sister, I get to live together. I get a break from my parents, and we get to hang out more."
"But wouldn't it be weird, for you us to be siblings when we're dating. Like it would be incest, wouldn't it?"
Jenny smiled. "It would only be incest if we actually do something sexual. Like we can just hang out together like a brother and sister would. What do you think?"
"I guess we could try. It's not like it's something permanent."
They opened Jenny's profile. They found a relationship tab, and there they could change Jenny's relationship with Jeff to 'siblings'. A few options appeared. The first one was who would join which fanily. They selected Jenny to join Jeff's family. Next option was about age. They were 5 months apart in age, not something feasible if they were to share the same mother. They could change one of their ages to make it congruent.
"Shall we be twins then?" Jeff asked.
"Nah," Jenny replied. "I want to be your younger sister. Hmmm. How about 14?"
"Sure. I guess." Jeff edited Jenny's new age to be 14. "Anything else?"
"Yeah. I saw something. Mental changes? Can we try that?"
"That sounds a little scary, but we can see what it's about." Jeff selected the mental changes tab. There they could see various info and options.
"Let's try it!" Jenny said. "It's probably going to make it feel much more real. How about sliding our mental changes to 30%."
"I don't know." Jeff was cautious.
"Okay then. 25%? 20%?"
"I guess 20% seems safe," Jeff gave in. He adjusted both of their mental settings. "Are we ready?"
"Yeah. I think so. Go ahead. Apply the stuff."
Jeff did.
He then looked towards Jenny. She was changing before his eyes. The most immediately noticeable change was Jenny's hair losing its red color, turning into a brown shade not unlike his own. Her facial features were different, a smaller rounder nose, a less narrow chin. She was starting to resemble him somewhat, or a younger version of his mom.
To Jeff's surprise, she grew a bit taller actually. It made sense with Jenny being a short 5'1", and Jeff's family being decently tall, but it was a bit counterintuitive with her getting younger. How tall was she now? 5'3" maybe?
Jenny's physique changed, but the differences weren't too overt. Her frame seemed a bit wider, and her chest seemed larger.
The changes seemed to have stopped.
"What do you think big bro? Do I look hot?" Jenny asked in her new voice.
"What?" Jeff felt a tinge of embarrassement. Jenny did kind of feel like his sister now, and it felt awkward considering that. He guessed the mental changes were in effect.
"Ooh!" Jenny exclaimed. "I can remember stuff about my new life." She seemed to be concentrating for a few momemts. "But it's not that clear."
Jeff also tried recollecting his new memories. Looking at Jenny, he could remember some events, or daily affairs, how he and Jenny left together in the mornings, or how she often came into his room, bugging him wanting to hang out and play, though that happened less after she became a teen. Though all these memories felt distant and sparse, and only barely like his own, which made sense considering they'd selected only 20% mental change.