 What is the flaw in the cousins' plan?
No one, least of all Martin, could dispute that a man's life should be guarded by his kin—but by those who hadn't been born yet? "He's dissatisfied with the current social order and killing you is part of an elaborate plan he's formulated to change it. It was then that Martin began to realize that either the whole lot of them were insane, or what Ninian had told him at first was the truth. And, on those worlds, it's far cheaper to use native labor than to ship out all that expensive machinery.
 They have kept Martin isolated for almost his entire life, he has no son. Therefore, they will cease to exist.
 Conrad could target their great-great-grandmother and achieve the same result.  Conrad could target their great-grandmother and achieve the same result.  They have kept Martin isolated for almost his entire life, he has no son. Therefore, they will cease to exist.  All Conrad needs to do to find Martin, is to follow the cousins back in time.