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He is really thin and athletic, unlike his great-great-grandfather, although he is almost the same in school - this may be the only characteristic he inherited from Nobita.
Sewashi's mother is only shown once, and resembles Nobita's mother, but more futuristic, a bit nicer and wears no glasses. Sewashi's father, who is Nobita and Shizuka's great-grandson has never appeared in the series.
It is known that Nobisuke II is his great-grandfather and that Nobita's parents are his great-great-great-grandparents.