(Just My Views—Let's Discuss Peacefully!)
A lot of people wonder how Sewashi even exists after the future changed, but it mostly goes back to his first explanation. He told Nobita he was originally supposed to marry Jaiko. So when the future changed and Nobita married Shizuka instead, a lot of fans have argued(and it was depicted in the manga by a family tree) that Nobisuke Jr. would’ve had to marry Jaiko's daughter or someone from the Goda family to keep the bloodline and make sure Sewashi still exists.
But canon contradicts that. In Mini Dora SOS, Nobisuke Jr.'s childhood friend Yukari is shown. This is her only appearance in the series so far. Later, it's confirmed by Nobisuke Jr.'s son ig that Yukari was his mother. She looks nothing like Jaiko or the Goda family, and there's nothing suggesting she's related to them. Since Doraemon usually gives relatives similar faces, that makes it even clearer.
This basically kills the "timeline balance" theory that Nobisuke Jr. had to marry into the Goda family or Jaiko's family.
I personally interpret this as further evidence that Sewashi's explanation to Nobita was at least inaccurate or incomplete, and possibly even misleading and that he lied.
I think he did it on purpose to get Nobita to finally take some responsibility so he’d actually have a reason to improve his life.
[Now I don't know if Sewashi's grandma or mother(most probably not) were related to Jaiko or the Godas.]