Ojisan's name is Shibazaki then? Both siblings have scary faces at a young age (lol) but the brother is clearly too tall for even the tallest elementary schooler. If they said middle school, then it'd be borderline plausible, since gowth disorders can make a middle schooler that tall.
page 26 : his name is not jiyan , but is jaian -> gian -> giant , or Gōda Takeshi .
page 30 : the nephew has been reading too much henta! , lol , some henta! 's plots go like this : a man will get a girl totally drunk , and then he take her home , and then he ......
page 31 : some more ideas for the channel of the "little" brother : shoot some friendly videos of him and his big sister , or of him and his short l0li / sh0ta friends , with his hand wraps around their necks .
the "little" brother must be a mutant . a few years later , when the "little" brother enter middle school , he will evolve into this guy , lol , in page 4 of this chapter : https://mangadex.org/chapter/78406/4
@Draggador probably not because he's the son of uncle's sister but considering his parents are divorced it might be his mother's maiden name so yeah plausible. I also saw a post in facebook about tall elementary schooler but granted her parents are both 180-90ish