I won't go into the details of genetics, but first of all, marriages between cousins are allowed in Japan. Secondly, the chance of genetic defects depends on hundreds of factors, ranging from genetic mutations in the parents and ending with how genetically diverse the community in which the child's parents live is.(In Japan, it's bad for these people, they are a rather closed dense society and there may be genetic defects even if the parents are not related at all.)
So for Kirito and Suguha, it's not even incest according to Japanese law. But with a child, it's a lottery. But we will pray for them.
View attachment 4183227