Your in laws became carriers through random chance of abnormal genes passed on through their genetics. It is absolutely nothing that they physically did. It is a rare genetic disorder and therefore passes through families. Do you brother and sister in law have the disease or are they carriers as well? Since your in laws are only carriers, they should not express the disease, but since two carriers happened to find each and have children, their children have a chance of either being fine, being a carrier, or having the disease. Again, it is rare, which is why you probably do not know of anyone else with the disease. Someone can be a carrier, but unless they have children with another carrier and produce a child with the genetic disposition for the disease, they may never know that they are a carrier. It is a strange twist of luck that both of your in laws carried the abnormal gene.