In case you didn't know, the real reason so many of the Robot Masters share the same color pallette is simply that the original NES was only capable of showing so many colors on the screen at the same time. This meant that, in additional to black, white, and the three colors for Mega Man, there were only a handful of available colors left, not counting any colors in the backgrounds.
Of course, if the same color pallette does indeed indicate sibling-hood, then I guess Metal Man is also a brother to Bomb Man, Elec Man, and Heat Man. Though, wouldn't all the Wily-bots technically be brothers? By the way, I wouldn't count Ring Man and Pharaoh Man, since they weren't built by Dr. Wily.
And while I'm thinking about it, how could Metal Man possibly know that Flash Man's Time Stopper kills Quick Man? Did Dr. Wily do tests where he applied different weapons to his Robot Masters to see how they react? And if he did do these tests, why didn't he correct the problems? Seriously, why build all these robots with these weaknesses? Hmm... maybe he included the weaknesses on purpose, in case they went rogue and... uh, started working for the good guys?