Sewashi is the great-great-grandson of Nobita Nobi. He came to the present world with Doraemon to help Nobita with his future problems including Nobita marrying Gian's sister, which are now solved and he will marry Shizuka Minamoto. Sewashi was the first owner of Doraemon.
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.
Not much is know about Sewashi's relatives than his great-great-grandparents, Nobita and Shizuka.
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.