When was incest first mentioned?

Although there are reports that adult incest has been notable in many royal dynasties, the evidence usually put forward has been subjected to much criticism. There are cases of siblings marrying which are verified.(And there are many cases in which first and second cousins married, a practice that would be considered "incest" in certain cultures today, but which of course was normal and non-incestuous when practised.) A motive often given by others for this supposed custom of royal incest is that this was in order to help concentrate wealth and political influence within the family. It is noteworthy that this motive is something attributed to these dynasties, not something that they themselves put forward. Since these dynasties did not, in fact, have the norm of royal incestuous marriage, it is specious to attribute any motives to a practice which didn't actually exist. Though usually frowned upon by present-day people, incest within families of royalty or of high esteem was done because the families believed that anyone who was not of their family was not worthy of marrying them.