That might be normal for the Japanese peasantry on their tiny island, but it's even more prominent in European royalty, where their level of inbreeding was so bad that one king's autopsy allegedly found that his brain or heart looked like a roasted walnut.
The Hapsburg autoposy.
One of the most inbred nobile lines that existed. that particular branch died off, but there's a bit of it left in the Windsor royal line.
You can GENERALLY get away with cosanguinous (cousin) relations a few times every couple generations without
TOO much trouble out side roughly a doubled chance for negative recessive trait expression and a marginally higher rate of birth defects, but the Hapsburgs did it for GENERATION UPON GENERATION. Sibling relations are similar, but the negative trait expression has an even higher chance and the rate of birth defects is higher too. again, you can get away with it once OCCASIONALLY, but the inbreeding coefficient is a BAD THING to fuck with unless you are deliberately going out of your way to LOWER it (Outbreeding)
Biology has a nifty little psychological trick to cut incest off at the pass too. The Westermack Effect essentially triggers a "This is family" response to people's you grow up around, which causes them to not be viewed as suitable partners. It also why stories of siblings who grew up separately and met later in life and only being grossed out AFTER the reveal is a thing, Westermack didn't trigger... it's also why someone you want to date saying "You're like a sibling to me" is so bad, you got Westermack'd.