I'm aware a lot of royal families practiced inbreeding for practical reasons, but I was wondering if there was ever a culture in which incest wasn't a taboo generally speaking.
By the Roman period, brother-sister marriages were quite common in Egypt among non-royals as well. This is basically the only known case of sibling incest as a normal cultural practice: http://www.jstor.org/stable/178753?seq=1#page_scan_tab_contents (requires JSTOR access for the full article, but the preview lays out the evidence pretty clearly)
Also, at one point, Zoroastrianism promoted incestuous marriages as well (parent-child as well as brother-sister), although how common incest ever was in Persia in practice is more debatable: http://www.iranicaonline.org/articles/marriage-next-of-kin
(Modern-day Zoroastrians don't believe in that stuff; "next of kin marriage" in modern Zoroastrianism only refers to marriage to a cousin.)
/r/AskAnthropology or /r/AskSocialScience might be a better venue for this question.