 Why don't the cousins realize the flaw in their 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? Nobody in the neighborhood was surprised when Martin's mother disappeared and Ninian came to take care of him. He'd been a war baby, born of one of the tides of soldiers—enemies and allies, both—that had engulfed the country in successive waves and bought or taken the women. Besides, if she really was his great-great-grand-daughter, as she said, why would she tell him to call her " Aunt Ninian "? "He's dissatisfied with the current social order and killing you is part of an elaborate plan he's formulated to change it. It wasn't that she cared what these people thought of her, for she made no secret of regarding them as little better than animals, but she was shy of attracting attention. From time to time, there were other visitors—Uncles Ives and Bartholomew and Olaf, Aunts Ottillie and Grania and Lalage, and many more—all cousins to one another, he was told, all descendants of his. The adults obviously figured that if a one-car family hired private tutors for their kid, there must be something pretty wrong with him. Though they were in the house with him, in their minds and in their talk they were living in another world—a world of warmth and peace and plenty where nobody worked, except in the government service or the essential professions. There was no devastation, no war, no unhappiness, none of the concomitants of normal living. 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 Conrad really got upset over the way Earth has been exploiting the not so intelligent life-forms on the other planets," Raymond continued. We're virtually the same people ... although it is amazing what a difference two hundred odd years of progress and polish can make in a species, isn't it?" 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. But he wasn't, for by now he had little respect left for any of his descendants. Perhaps a great-grandfather is responsible for his great-grandchildren, but a great-grandchild can hardly be held accountable for his great-grandfather."
 They are not very intelligent.
 They do not understand time travel.  They all originated from the same point in time.  They are highly interbred.  They are not very intelligent.