This is not so much a question to historians, as one to behavioral biologists, since incest is also a "taboo" in many animal species, or rather the instinctive logic of "don't breed with it if it smells like me". So it also remained a "taboo" in primitive people before the appearance of the concept of royal blood and other cultural phenomenons, such as the holy Zoroastrian marriage.

According to Claude Levi-Strauss in The Elementary Structures of Kinship, the taboo surrounding incest developed due the necessity of "trading" daughters to nearby villages. Groups which practiced incest risked losing valuable alliances with other nearby groups, and the consequent benefits of such alliances.

This is more of an anthropology question. There are incest taboos in every culture throughout the world, though what is taboo, varies. Like, one culture will forbid an uncle from marrying a niece, while in another culture, that's fine. One culture might find the marriage of first cousins to be ideal, while another might find first cousins to be ideal if they're cross cousins, but taboo if they're parallel cousins (cross cousins are first cousins whose parents are brother and sister. Parallel cousins are first cousins whose parents are siblings of the same sex).

So while the definition of incest might change, the idea that you shouldn't marry close relatives exists everywhere, including that you shouldn't marry a parent/child or sibling.

BUT: the rules were different in just a few societies in the case of royalty. Like in ancient Egypt. It was not acceptable in that society for siblings to marry... except for royalty.