Quiet often you might have come across the stories of identical twin cousins born 20 minutes apart. They could marry two identical twin sisters and had a couple of baby daughters 20 minutes apart. This would make you put the thinking cap on for sure. Your brain might be in a state of confusion. What would be the case if a couple of identical twin brothers went on to marry identical twin sisters and went on to have babies. Such a marriage goes by the name of quaternary marriage and till today there are about 250 cases existing in the whole world.
Let us explore this situation as identical twins are expected to have the same genic material. It means that the twin brothers along with the twin sisters are identical. Suppose one of the brothers has a baby with one of the sisters. Now this position is repeated with the other brother having a baby with the second sister. From a genetic point of view, these cousins could be identical twins. Yes you have got it right as cousins that are identically genetically. Let us move a step further. How could you assume that it seems to be virtually impossible? It would be asking too much as how siblings that are born from the same parents can be identical. There are chances but it would be almost next to zero.
It has been proven that cousins of a quaternary marriage work out to be genetically matched siblings. What would be the situation in such cases? First and foremost with the help of a DNA analysis it was not possible to distinguish who the parents of the baby are. Just be aware of the fact that the babies are identical. The moment you test the DNA sample of a baby against the set of other parents it could match to both. As per legal laws in some countries of the world, cousins can marry each other. But what would be the cases if the cousins are also siblings. Till now there is no legal protocol that has been able to answer this question.
Considering another angle since both the brothers and sisters are identical twins they could virtually be having the same DNA. This cannot be the exact case but almost come close to it. The reason being that both of you emerged from the same fertilized egg. For this reason the identical twins go by the name of monozygotic. As both of you are having the same DNA it could be a two sets of children having the same mother but dads will be different. From the DNA levels they tend to be more like half siblings and not their first cousins. Instead of the usual 12.5 % they do share 25 % of the DNA. From a legal point of view the sisters kids and your kids are first cousins but from a DNA perspective they work out to be half siblings.