>>37604Cousin marriage is. A brother and sister would still get their heads lopped off in Saudi Arabia, hanged from a crane in modern Iran, and face either some form of execution or extensive prison time in any country based on Islamic Law for having sex with each other. Of course, how much even this strict prohibition is adhered to behind closed doors in these countries is an open question, but nonetheless…
Sociologically, cousin incest and sibling incest are very different phenomena. Cousin marriages are historically common even if there is variance in their frequency depending on time and place. They are very frequent among clannish cultures as different family branches marrying amongst each other strengthens extended kinship ties, and there is a strong inherent distrust of anyone outside the clan. Sibling marriages were rare, happening most often in the context of dynastic practices exclusive to royalty or nobility. Which is why the few known occasions when they happened as a general practice across all social strata are remarkable from an anthropological perspective. But the most well documented of those occasions, that being the aforementioned Ptolemaic and early Roman Egypt, doesn't offer any significant clues suggesting that the widespread brother-sister marriages there were driven by the same social forces that motivate cousin marriages elsewhere.
So it's important to distinguish between the two for accuracy's sake. Biologically very similar as then it's a simple difference of degree, but socially quite distinct.
