I guess that going away at the crucial time is their way of not facing up to what they know to be the truth, which is that your brother is a winner and they are sad losers.
I also suspect that your df, in particular, is secretly green with envy and wishes that he'd had the ability, the foresight, and/or the drive to get himself a degree or further his education in some way.
No harm in pointing out to them that by failing to attend his graduation they have spectacularly let your db down, but it's not likely to make a visible dent or bring about any change in their attitude.
It's probable that your db is in for years of snide comments about his poncey non-job, but he shouldn't let this get to him because he's on a fast track to material success and, in all the richness and fullness of his future life, your dps will be a very minor cross to bear.
I hope you'll move heaven and earth to be with your db on his big day - tell him that anyone with any sense will recognise his superb achievement in getting a first, and those that can't see what he has already achieved in his young life are to be pitied.