Trying to find non-repetitive ways to describe diapers and the things surrounding them was something that I struggled with when I tried my hand at writing a fapfic as well. Sometimes there just really is nothing you can do. Same goes for trying not to overuse I, me, he, and she when you the entire story is a small group of people interacting.
However, Zack does need more character. He can try to play it off as normal while still being playful or sweet. Just something other than assembly line diapering. It would also help to have the MC show more reaction to him as well. Shes supposed to like this guy and she starts the story in the same room as him, half naked, watching him diaper his sister.
Maybe shes jealous cause she wants to be pampered by him. Maybe shes daydreaming about laying down on his lap. We are in her mind and sharing her perspective yet we get so little of what she wants and thinks. What about when shes laying there bare naked in front of him? Does her stomach tense up? Heart flutter? Maybe she gets turned on, depending on how sexual you want it to be. Just some things to thibk about.
Anyway, its beddy bye time for me.