last page 's announcement : " although this may look like a side chapter at the end of a volume ... "
the reason is because at the ends of some other manga-s 's volumes , they often include some side chapters / omake chapters / filler chapters / fan-service chapters about the beaches , the hot-springs , etc , etc , ....
Uncle was trying to erase the girl and the little brother's memory after they saw his magic, although the nephew and the niece-in-law-wannabe stopped him.
As for the nephew.. maybe something like trying to clean impure thoughts?