This could actually be possible considering nothing before Humphrey's "pre-teen" self is shown. Maybe he just forgot it, whoever is older left their parents before the other was born, or Tony just doesn't want to speak of it. Of course, in order to check we need to know and see both their parents and use a punnet square to check all the different combinations of ears, eye color, etc. to know if the can be related. Unfortunately, we don't know Humphrey's parents or Garth's mother.
Actually, they are brothers since Humphrey married Kate and Garth married Lilly, making them a brother-in-law. And if Humphrey was indeed Garth's brother, would he have recognized Tony as his father, so that takes the possibility out. But either way, they are both brothers
