During the Graeco-Roman period, sibling marriages were common among all classes in Egypt

One of the famous relationships of Egyptians was the marriage of Cleopatra VII and her younger brother, Ptolemy XIII. Their parents, Cleopatra V and Ptolemy XII, were also brother and sister. Sibling marriage had been an old tradition among all the rulers of Ptolemaic dynasty starting from Ptolemy IV and was done in order to keep the Ptolemaic blood “pure” and “to strengthen the line of succession.” The ancient Egyptians believed that royal women carried bloodlines and so the ruler marrying a sister of half-sister was thought to be advantageous. Apart from the royalty, there are also accounts of several of the Egyptian pharaohs marrying their siblings and having children with them.

 

Egypt(mostly in royal and noble families) and Persia(religious culture) were two of the last societies where incest was socially acceptable. Before them incest was acceptable in many more societies, specially between siblings. At first, it was acceptable for all people, then it stayed only in royal/noble families. And then it started to become illegal because of excessive inbreeding in same families, then it became a social taboo and so called immoral for not practicing openly anymore for thousand years. Modern religions made this practice more taboo. Now only cousin marriages/relationships are normal(most of Asia and some European countries). But the good thing is, more countries are accepting cousin incest nowadays. Hopefully in future other types will be accepted too. Because that reason why incest became illegal isn't a big deal now, you can have sex with protection easily and avoid pregnancy if you want. And the other problems of incest are mostly social problems caused by the modern social construct in which incest is an immoral act, those are not actually problems caused by incest itself. People who do it willingly, some of those face problems mostly because of the social pressure. It was same for gay people in western countries even 20 years ago. So consensual incest still being a taboo is logic-less in this century.

 

Well! it will take time. But it will be accepted in future for sure. Just compare ten years ago with now. People wouldn't even talk about this back then. Nowadays more people are taking this softly, and you'll find more open minded people who are attracted to their family members and wanting to have this kind of relationship. Though it's in the very early stage of normalization process, still it's something.