On the other hand, if we met Seth’s brother, that would be easier to pull off for Jeff Goldblum, as this brother could easily be explained away as a twin.
The Fly II followed Seth Brundle’s son, Martin, who developed at an accelerated rate, was played by Eric Stoltz as an adult and ultimately followed in his father’s footsteps by turning into a fly-like monster.
Unlike Seth, however, Martin managed to revert to a fully human form, with the sequel’s villain, Anton Bartok, being turned into the monster.
As for why he would turn into a monster, well, maybe he just didn’t just take what happened to Seth seriously enough and subjected himself to his own experiment, or he was transformed against his will.