Even if there is a low chance of deformation, there still remains the chance of deformation. Also, if there is a higher amount of inbreeding (people marrying first cousins), the genetic congruence will become more and more similar and rapidly increase the amount of deformities. Procreation with those unrelated to you ensures a healthy amount of genetic mutation, whereas inbreeding ruins the gene pool.

Also, why those people sit there and whine about how they should marry their family is beyond me. Seriously. Grow some ****ing nuts and find someone else.