Sad, I think.
I can understand why they broke up in DS days - must not be easy to exist next to such a creative force as Mark.
Yet, in 1983 the two broters seemed reconciled as Mark played on David's first album... but this was never done again. So there must have been a second break up. Maybe because of the massive success of the BiA album and DS going into the pop direction, which David may have been disgusted with ?
Anyway, over the years, Mark and David's philosophies seem to have come closer: more initimated music, no more stadium rock, etc. So, why no relationship after all these years ? Maybe because, as I think DK mentionned once, he considers that Mark has done charity concerts to promote his own album.... Still, that's not a valid explanation.
If Roger Waters and David Gilmour, after 20 years spent trowing their toys at each other, can now play together and hug each other on stage, why can't the Knopfler brothers ? Is Mark that strange of a guy, as David hints in this interview ?