If two pairs of identical twins have children with each other, wouldn't their children be biologically equivalent to siblings though they are technically cousins?

They would not be exactly the same because of epigenetics. Identical twins come from the same embryo therefore have the same DNA (initially). However, whenever you have a child, or when your cells copy themselves, the copy/paste job is not perfect.

Things get mis-copied and change along the way. Your DNA can also be changed/damaged while you are alive, eg by carcinogens that interact with DNA. The more it changes while replicating during your lifetime, the higher chance you have of developing a cancer, since parts of the instructions that program a healthy cell can be lost. The concept of DNA replication errors also applies to the transfer of DNA to a child, so 'typos' are made and the output is ever so slightly different. Also how evolution works.

Additionally, different proportions of the parents' DNA is fused together taking different parts from each, otherwise all siblings would be essentially identical too.

/r/NoStupidQuestions Thread