He thought about it, but then realized he had the perfect target waiting. "This will go great.... I think. But first," He went back to the Chronivac and made a small change so that his actual penis was what was used. "That'll work better."
He went out of his room to find where his sister went. "She can't have gotten far." He checked the kitchen and living rooms, before heading down the hall to her room. "If she's not getting a snack, she's probably in here."
He walked into Alice's room, and didn't see her. "Huh, she's not here?" He went to leave, but noticed her bathroom door slightly open. "Unless..."
He went over and opened the bathroom door, and found Alice sitting on the toilet. She didn't actually seem to be using it, though, and she didn't notice him walk in. He quietly walked up on her.
He decided to do this more sneakily instead of his original plan. Dropping his clothes behind the toilet, he slid inside of it carefully, successfully making it under her. As he thought, she was just sitting there, not even using it. "What is she even doing..."
He thought about if he wanted to just shove himself in there now, or use his penis to open it up; Deciding it may be tight otherwise, he decided on the latter.
He positioned his penis underneath where her vagina would be, and pulled her in, causing her to scream as he did so. She landed right on him, and he kept her in place so she doesn't move or see him.
She struggled in his grasp as he kept breaking the seal, and he couldn't help but find her struggling funny. He stayed as quiet as he could, keeping everything in his thoughts as he kept on going in. It felt surprisingly nice, and he decided to do it a bit longer than his initial intent. "I think that's enough."
He pulled out of her, his cum dripping out and sticking around, and let her go, the result surprising him; Rather than jumping out and looking, she pulled herself out and sat on the toilet again, this time actually using it. "Well, I guess now's a good time as any..."
He knew Alice had always wanted to be a boy, and that she wanted to know what it was like to have a penis. She tried to keep it hidden, but he could tell ever since she was a toddler. Even now, she always blushes and tries to pass it off if he mentions it, but he can see through her and tell. So now that he has the Chronivac, he wanted to grant her wish.
Trying to avoid her stream of pee, he rose up and pulled her labia open, aligning his head to enter. Taking a breath, he slowly pushed his head through the flaps, feeling the flesh of her vagina try to stop him at first. But he didn't get in all the way; He felt it pull away from him, as Alice screamed.
He looked at Alice, who was now standing and looking at him as she questioned him. "Jeff, what are you doing??"
He simply looked at her with a straight face, and decided to just get to the point. "Well.. I've always known you wanted to have a penis, so... With the help of this cool machine thing, I wanted to fulfill your wish, and get closer to you at the same time. Besides, it seemed like you wanted to be pretty close; You didn't seem to care while I was in there."
She stared at him, and he couldn't tell what she was feeling. "I thought you were just trying to play a weird game! And what are you even talking about, machine thing?"
He decided not to answer, and instead to pick her up and pull her towards the toilet. "It'll be easier to just do it than to explain, but just sit back down, continue using the bathroom if you want, and I'll do my thing."
He could see her face was a bright red, unlike it's natural white color as she came a bit closer. Jeff lowered himself back in as she sat down above him. He put his face right in front of her vaginal opening, turning around and attempting to get in. His full head went inside, and he pushed forwards, using his own cum to slip right in. He could feel the Chronivac taking effect, aging him down as it shrunk him.
He slowly went further and further; His shoulders went in, then he got down to his elbows, before getting his entire upper half. His head entered into her womb, which was filled with his cum from before. His lower half hung out, Alice's pee sliding on down his younger body.
Compared to what he originally expected, he found it really comfortable inside her...
-------- Alice's view --------
Alice was breathing heavily; "Did he actually... What did he even... He's inside of me..." Alice lifted up her skirt to look, and saw the legs of someone around her age sticking out of her vagina, with a penis on the edge of her labia. In shock at what happened, she reached down to feel it and see if she's dreaming. "He actually..."
The legs were moving around, but they stayed somewhat spread and inside the toilet; They were a bit sticky from Alice's pee flowing down them, and Alice just ran her hands around her new genitals. "I can't believe this... It's actually... happened.."
She held her brother's penis in her hand, and took in the feeling. "I don't know what to even do..." She rubbed her hands around it, and could feel it harden out a bit, sticking out more.