 What is the flaw in the cousins' plan?
Conrad's idea had been to go back in time and "eliminate!" In that way, there would be no space-drive, and, hence, the Terrestrials would never get to the other planets and oppress the local aborigines. "I wormed the whole thing out of him and all of us—the other cousins and me—held a council of war, as it were, and we decided it was our moral duty to go back in time ourselves and protect you." Induced , Martin knew, could have meant anything from blackmail to the use of the iron maiden. If one of us guarded you night and day, he would never be able to carry out his plot.
 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.