it was common in almost every single ancient civilization with no exception especially amongst royalty. You can look up ancient Japanese, Chinese, African, etc.
What is right and wrong is an every evolving concept. But, it is normal that us, who live in the present, might feel icky seeing this portrayal even in fiction
yeah, I think that a lot of people don't realise how period-specific values and ethics are. I actually only remembered the bit about Japan and China after reading your comment and wanted to add my two cents. I went down an internet rabbit hole a few months ago and it led me to the bizarre practices ancient civilizations had and how sometimes they make exceptions for nobles and royalties when it comes to incest even when it is looked down upon amongst commoners

I get that they mentioned it's natural in that world to marry half siblings also because of the quality of magic one holds but man this ain't for me gbye