Diana gets up and rushes to the bathroom, desperately trying to hold it in. Whether she remembered that Jeff was there or not, Jeff didn't know, however he was glad when Diana pulled him down to sit on the toilet.
Jeff being Jeff decided to watch Diana pee, however, as she pulled him down just enough to let him watch. He was mostly curious, as he hadn't seen a girl's privates up close when peeing before, and he had nothing else to do. So, he watched her vagina as she peed, taking in the sight; Smooth skin with pee coming from a small narrow slit.
"That was too close," Diana had said, talking to herself "I don't want to ruin this cute diaper already!". Diana pulled Jeff back up along with her panties, before leaving the bathroom to continue playing with Molly.
In the mean time, Jeff wasn't feeling as bad; He was starting to enjoy being his sister's diaper. Although he has to deal with the knowledge that it's his sister, and that his arms are being stretched, it could be worse. He could be on Molly instead.
(Time skip, 15 minutes later)
From what Jeff could see from under Diana's dress, Her and Molly were once again playing with dolls. Barbie dolls, to be specific. While Jeff was watching as much as he could, he started to wonder; "Why do barbie dolls even exist? They don't even look like they're appropriate for children..."
At that moment, Jeff's mom came back.
(Conversation key: M = Jeff and Diana's mom, D = Diana)
M: "Diana, you ready to go?"
D: "Yes!!"
M: "Well aren't you excited! Though, do you know where Jeff is?"
D: "No, Haven't seen him all day. I looked in his room earlier and he wasn't there!"
M: "That's odd, he's normally in there. Maybe he's with some friends or something."
After that, They left, not knowing Jeff was tagging along as a diaper. And as they drove off, Jeff was once again left in darkness, aside from the view of his sister's privates.