Biological Alarm? If that existed, people would've stopped eating fast food. People do lots of things that are bad for their health because it simply feels good. Biological alarm cease to go off when there is an animal attraction between two people, now it really doesn't matter what gender or relationship they might've had.

What turns most people off about their siblings is the fact that they've lived together since childhood and suddenly feeling different about someone rarely happens compared to feeling love and attraction at the first sight.

In theory if people bred with their siblings, carriers of defected genes would've ceased to exist, due to the fact that their next generation would've not survived while those with perfect gene pattern would've produced generations of more consistent and stronger genes. However many gene patterns would've ceased to exist as the result.

With the existing taboos, carriers of defected genes spread up more and more and chance of the same defected genes paring up increases in every generation more than before. Here, pre-marital tests on thalassemia and hemophilia are mandatory. I personally know two people that were engaged but didn't get married after getting their test result, and they were not even remotely related.