In Japan cousin marriages mostly occur between the wealthy elites instead of rednecks (although landowning farmers also tend to practice it), so the stigma associated with the practice is reversed. It's seen as a legitimate way to maintain wealth in the family. In some of the outlying islands, especially in one-clan communities, it's basically inevitable that cousins will marry each other. The gene pool needs to be refreshed once in a while, but the simple fact is that multiple generations of cousin marriages don't have such a detrimental effect if the family gene pool is already large to begin with. Japanese families used to have many children and cousin marriages still mostly maintain the same surname taboo, which is basically what we see in this manga. If Shimada married his cousin from the Saito family and their child married her cousin from the Iijima family, then the Iijima's child married his cousin from the Mishima family, then the consanguinity between the next Iijima and the next Shimada is far enough apart for them to safely marry despite the fact that they're cousins. Note that only four bloodlines are needed to make this work, and in fact four-clan alliances were common throughout Japanese (and Chinese) history.