He decided to see who he came across first, and have that be his target. "This could be fun, depending on who I get!" So, he walked out of his room, and started looking around.
After a few rooms, the first one he saw was his younger sister. Peeking in the room, she was playing with her brother, with stuffed animals spread out. He had his decision, and while he could've backed out, he decided to stick with it. "I just need good timing."
He sat there outside the door for a few minutes, trying to stay out of sight. He knew he found his chance when he heard "Sorry, Izzie, I gotta go potty!" from in the room. He saw his younger brother running towards the bathroom, with Izzie sitting there, curious for a second before going back to her toys. "This is it!"
Jeff quickly took his clothes off, hiding them in a nearby doorway. Sneaking into the room, Jeff made sure the bathroom door was closed before walking to his sister. He carefully pulled the waistband of her skirt back, before her diaper as well. She didn't think anything of it, having been used to random checks for the entire five years of her life.
With his position, he hopped right in. He slid in pretty easily, despite being a lot bigger than Izzie, let alone her underwear. The waistband closed behind him, and he was fully inside. After adjusting his position slightly, His face was pressed against the soft lips of her genitals, with his penis and legs against her buttcheeks.
Going into this, he thought he'd regret his decision as soon as he saw Izzie. But, Jeff finds it surprisingly comfortable in there; Her legs act as a soft pillow on either side of his head, and being this close to a girl is turning him on, despite being his sister. He almost felt as though he could fall asleep here.
He could feel Izzie shifting around, but overall continued with her toys.