Sure, incest has quite the storied history. It's fairly well-documented, especially in places of nobility and cultures of "pure blood". That said, incest has been taboo since humans have had laws, really.

There's a well-known behavior among humans called the mere-exposure effect, whereby you become attracted to people you spend a lot of time with. Generally the first thing that triggers a new romantic relationship is time spent together, and conversely a common trigger for breakup/divorce is lack of quality time together. Now, extrapolate that to family, and you have a problem in which you have a high chance of becoming romantically involved with your progeny, caregiver, or sibling. Luckily, humans have a pheromone/mechanism (not sure exactly, read about it once and I dread the search to find it again) that short-circuits that behavior and generally causes you to view immediate family as non-mating partners. Obviously this isn't perfect either, because there are plenty of stories about familial rape, but it's part of why the practice isn't universal. On the contrary, the occurrence is low enough that we might consider the ones who partake to be aberrations.

As always with humans, it's complicated. There's a mix of primitive, biological urges, as well as societal pressures to keep healthy gene pools flourishing, but it doesn't stop people from being tempted or indulging in behaviors others might seem unsavory.