@Zuccman
The reason people in the old days did not care about incest as much as we do today and why the village is not full of mutated cripples is because it really wasn't that bad.
Ok hear me out i know what your thinking it killed alot of people just like you said and that is true....
But the problem is that back in those days EVERYONE had like 5-10 kids because about half of them died to normal sickness anyway. Even the richest of the rich
also the whole deformity from incest is greatly exaggerated its up to luck and is
alot less common then people think.
first off you have to actually get affected. and as i said its not as common as people thinks but let's go on for now....
then the birth defect have to be something negative as getting white hair is not actually a problem for your life unless people are superstitious... oh wait they were...
genetics have random mutation all the time in healthy people and the body is made to fight off anything that it deems harmful. That alongside survival of the fittest is why evolution is a thing.
then it have to actually be a severe case to the point it actually cripples you as for people to actually notice.
You getting a little worse eyesight, getting 5 centimeter shorter or getting an arm thats 3 degrees tilted so it grinds up your bones faster is not going to be noticed in the long run off your life.
and you know.... those things can be because of everything from lack of food to poor medical and health practises in general.
There were alot of people who got sick and died from only eating the same type of food everyday. scurvy( lack of fruit) for sailors or rich man's disease(to much meat) for nobles are perfect examples.
then finally the kid who is affected by a birth defect also actually have to survive and get kids because again half the people died from sickness or injuries anyway.
Only then have he pass on that corruped genetics. note that all genetic information is not passed on in the first place. and only a verry small numbers of mutations ever get hereditary and gets passed on which is part of why it was rare in the first place.
now as a final note i want to again state that inbreeding is a bad thing, but so is getting a cold.
getting a cold nowdays is nothing to care about, you are in bed for 2 days and you are good as new and wont get sick for a long time to come.heck lose both leg and you just need a wheelchair.
but back in those days getting a cold ment that you, 2 of your brothers or sisters , and the neighbour next door who delivers eggs every week to trade for your milk or bread dies a horrible and painfull death and losing your legs meant you had to pray your parents had the money to aford drugs so they could kill you in your sleep as keeping a cripple alive was a huge problem for anyone but the richest.
so yes inbreeding is a real risk but it really wasn't all that bad if you look at the bigger picture.