Even when brothers and sisters have children together, there is very little risk of genetic problems unless the problems are already present in both brother and sister. People think that incest "creates" genetic problems, but that is impossible and shows a lack of understanding of human reproduction.
When two people have a child, the child gets 50% of its genetic material from the mother and 50% from the father. If both the mother and the father have the same predisposition to a certain disorder or disease, the child has a much higher chance of having that disorder or disease because it is already present, NOT because incest in and of itself "creates" genetic problems.
But if both the mother and the father are healthy, the child has a much higher chance of being healthy. This is true whether the parents are related or not. If the parents are related, creating a child "doubles up" on the shared genes that are already there, but that cannot "create" problems out of thin air. It will only magnify any problems that are there, but it also magnifies any good qualities which are there.
This is one of the reason royal families are often so inbred (usually cousin-marriages and half-sibling marriages) and so long-lived and healthy. There are other reasons too (such as access to healthcare) but this one shouldn't be underestimated. Royals have practiced incest in varying forms for centuries. This is where the phrases for "cousin," "first cousin," "second cousin," and so on come from (I have a degree in linguistics). The royals recognized the benefits of inbreeding carefully.