 Why doesn't Martin explain the flaw in the plan to the cousins?
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? Nobody in the neighborhood was surprised when Martin's mother disappeared and Ninian came to take care of him. "He's dissatisfied with the current social order and killing you is part of an elaborate plan he's formulated to change it. One day the truant officer came to ask why Martin hadn't been coming to school. But Ninian didn't know that and she went into a real tizzy, babbling that Martin had been sick and would make up the work. Martin was told to call him Uncle Raymond. 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. When Martin was sixteen, Raymond took him aside for the talk Ninian had promised five years before. "And Conrad really got upset over the way Earth has been exploiting the not so intelligent life-forms on the other planets," Raymond continued. And, on those worlds, it's far cheaper to use native labor than to ship out all that expensive machinery. I don't mean in the now for me, but the now for you," Martin explained laboriously. "I'm trying to talk to you as if you were an adult," Raymond said, "but if you will persist in these childish interruptions—" "I'm sorry," Martin said.
 Martin does not want the future generations to turn out like his descendants.
 Martin resents the cousins for taking Ninian away from him.  They have been very generous. Martin is afraid they'll leave, and he won't be wealthy anymore.  Martin does not want the future generations to turn out like his descendants.  Martin finds the cousins very irritating. If they can't figure it out, why should he explain it?