Is cousin-fucking a thing in Japan? Or just in Yaoi?

Worldwide

While I'm sure cases of incest occur in most countries, I mean as an accepted thing? I've noticed the cousin on cousin thing in straight manga as well. In western countries it's illegal, or heavily frowned upon if you're second cousins or closer. My great grand parents were first cousins and all their kids had f*cked up genetic problems, mostly mental illnesses and a few physical issues.