Right. So if I have two brothers, one is President of the U.S., and the other is a garbage truck driver named Fred, I would write down that I am the brother of Fred, not the President of the US ?
Recall that the brothers mentioned in the gospels ... were late-comers to the party.
James and Jude didn't believe until Jesus appeared to them post-resurrection ... and that they were quite derisive of He and His ministry before that point.
I would think that, in consideration of those facts, they would not feel as comfortable on claiming relation with Him (even though they were related) ... in their writings.
I don't see how that effects what Jude wrote years after the resurrection. As a matter of fact, it would give more force to his writings by making the more direct claim (if true).
Regards