"Jeff? How did you... How are you so small? And what were you doing down there."
Jeff, unaware of what to say, stutters out "I-I don't know! I was sitting on m-my bed, and then suddenly appeared in there!", silently praising himself for his last-minute lie. It was partially true though.
Diana thought for a moment. "I don't believe you. What were you really doing down there."
"N-Nothing! I just s-suddenly appeared there!" Jeff continued trying the partial lie, but Diana wasn't having it. "Really? Well, I'll have to keep you with me until you know." She suddenly pulled her diaper open, and dropped him in.
Jeff was suddenly forced into the darkness, the infant-sized diaper barely giving him space between the bottom and his sister's crotch. With Diana's size, he was stuck on all sides; Below him was the diaper, above him was her crotch, and to his left and right were her legs.
Diana sat down, sitting directly on Jeff's back. Due to Jeff's positioning, her urethra was aimed right at his face, as she thought of what to do with him in the event he doesn't cooperate.