Just from the title and pictures, the MC's older sister is hit in the head and mentally regresses to her toddler self. The MC seems to take advantage of this and fuck his sister in various ways.
I'm pretty sure that diaper tag needs to be removed. Unless a diaper is mentioned in the Japanese text somewhere in the novel, which I doubt seeing as there's no pictures to accompany it.
Diapers are in the text. Chapter title in page 141 / image 144. Suggesting it in 148 / 151. And putting it on, along with dressing her entirely on the next page, 149/152. Which ends with her being so happy just by wearing her uniform that she forgets about the diapers entirely. Fun stuff, really.
As far as I can tell that chapter is the only diaper one. But my reading is so painfully slow I'll probably never check it all through. Sadly, diaper is a visual tag according to the wiki, so it technically shouldn't be there. But it's nice to know.