Are babies of twins married to twins twins?

A friend posted on Facebook the above picture with the caption "2 twin brothers married, 2 twin sisters, resulting ..... Identical babies!!!

He commented his skepticism with the question "This can't be legit can it?"

While I don't remember much from my biology classes I do recall that twins can be either fraternal or identical. Fraternal twins are the result of two different eggs that are fertilized by two different sperm cells within the same pregnancy. So essentially they are normal siblings that happen to be born at the same time. Identical twins are the result of a single egg that is fertilized by a single sperm cell to form one zygote which then splits in half. The two halves (embryos) develop into babies that are nearly genetically identical.

The conclusion then to the question posed by my friend is that it is not possible for the babies of identical twin parents to be identically the same since they did not develop from the same embryo. Just as we are different from our older and younger siblings, these babies would also be different. But this leads to an interesting thought. Technically they would be considered first cousins but since their parents are genetically nearly identical to each other, wouldn't that mean that they are genetically siblings?

As strange as it may be, the answer is yes, the children would be as closely related, genetically speaking, as siblings born from the same parents. Below are a couple of articles I found which support this.




ABC Science: Do twins x twins = twins?
Popular Science: If a pair of female identical twins mates with a pair of male identical twins, will their kids also be identical?