Jeff had an idea, and decided to go for it. "This could either be fun, or backfire horribly." He minimized the Chronivac window, promtly leaving his room as he headed to where he knew his youngest sister, May, would be.
Entering the room, having avoided his other sister, he walked over to where she was playing. He picked her up, and set her somewhere more hidden from sight. He reached down and pulled her skirt up, before pulling her full diaper down, his sister looking cute and confused. He was about to enter, before deciding to do something else. "It may be a bit tight right now, and It'll be where I'm located soon anyways..."
He flipper her over onto her stomach, her legs hanging off the side. Pulling his pants down, he shoved his penis right into her butt, May squirming slightly as he started to pump. He was stopped quickly, however, as she clenched it hard, locking her in place on his penis. "Uhh, this may be annoying..."
Jeff kept pushing her forward, trying to at least get her off. He slowly slipped out, but the head was completely stuck; He let go, his arms tired as his penis went towards the ground, May hanging on the tip. He regained his energy a bit, before pulling her entirely off. "That could've gone worse."
He put her onto the ground, sitting in behind her and laying her on her stomach. He took in the view for a second, before the stench of her diaper got him; "I'll have to deal with that first, it's too much!"
Trying to go fast, he pulled her diaper all the way off, disposing of it and clearing the stink; He didn't grab an extra, as he didn't know how to put one on, however. He laid her down on her stomach, and positioned his feet near her anus. "Alright, it's time..."
He pushed forward, his feet forcing her butt open as they barely slid in. He could tell she was in pain, but he continued on, pulling her more. After a bit of pulling, the Chronivac kicked in, regressing him down and shrinking him as he entered position; He entered a bit further, his small penis right at the edge of her anal hole
After adjusting his position slightly, he could only chuckle at the thought of this look; Him at a young age laying on his chest, sitting with his lower half inside of his formerly-younger sister's butt. He partially regretted his choice due to the smell, but he had decided already. And now he decided to wait a bit