The Effect of Inbreeding in the Royal Family

We've had many questions about the ideas of in reading. So one from ivet righolt on fasfock and kath caspor on instagram, what was the effect of this inbreeding in the royal dynastries? And how much of it was there? Is it just speculation that there was lots of it, or do we know this for certain? It's absolutely certain that kings married their sisters and their half sisters, we know this. But it wasn't something they had to do, and not every king did it. i think there are probably enough breaks in m royal family that this happened for itd not to become an absolutely massive problem.