 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? 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. "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?