hi! you may find additional info in /r/AskAnthropology (or /r/AskSocialScience?) re cultural attitudes, or /r/AskScience re how quickly noticable genetic defects may appear, but here a few previous discussions from this sub to get started on:

Did people in medieval Europe ever link birth defects to inbreeding? If not, when was that link found?

What are the historical origins of incest as a taboo? Was incest always taboo? If not, how did it become taboo? What were the historical arguments for / against the practice of incest?

I've once heard an anthropologist claim that every human society ever had a taboo on incest. I know that there were some. Dry strict laws regarding incest in many places in the past. Were ancient peoples really so aware of the genetic effects of incest?

examples of culturally-acceptable incest: Brother-Sister Marriage has links to loads of posts, plus this more recent one Incest in Medieval European Villages

examples of birth defects: Has there been any royal babies with severe birth defects ( physical or mental)? How did they handle it?

How far back are we talking? And what are your parameters for incest? People all over the world settle down with their cousins.