Chatting yesterday with someone in an organization I do some work for, and he starts telling me that his son is getting married soon, congratulations and so on, and then goes on to say how happy his mother is, because the lad is marrying his cousin, who's also her grandchild. 

I know this is not unusual in many cultures but I realized I was at grips with an ingrained cultural taboo. I was relieved to find out that my acquaintance was not himself married to a cousin, but to a woman from outside his culture entirely. Hapsburg outcome avoided.