I'm not a geneticist, but I do know that people who are into breeding animals do keep a close eye on desirable characteristics, which can safely be enhanced by selectively breeding with relatives. They also have a good understanding about the limitations of such an approach.
I assume that geneticists actually do have a pretty good understanding of what kind of gene pool one needs to sustain a population. Especially when a single bull can artificially inseminate hundreds or thousands of cows, you need to know what you are doing.
Edit: apparently, that is the biologically most effective method. So perhaps games like WVM do get it right...
Yes, but if you look at family trees of noble houses, you'll see that that is actually a VERY small gene pool.
And, perhaps more problematic, marriages were more based on political motivations than genetic characteristics.
Or even pure lust (which arguably has a biological component) like in these games.
That is very interesting, thanks for posting! But it's also contradictory. Like this statement:
If it was really that bad, then there would be no way that they could have survived from the 14th century to today.
So, it seems that it does introduce some health risks, but it's manageable and does not damage the population as a whole.
Also worth to consider: in previous centuries (and even large parts of the world today) high child mortality was anyway a given and children with serious defects didn't survive.