@ilaya It's not uncommon for royalty and high noble houses to share distant family trees; nobles would have had to marry into the royal family to climb the ranks, and in turn having a branch family ensures that the royal family will have someone of "proper pedigree" to maintain the "purity" of the royal house (remember that many ancient civilizations had founding mythos that tied their monarch to divinity either by lineage or by ordination). It sounds weird on paper, but chances are enough generations have passed since Rize-tan's family became a branch that calling it incest would be a big stretch.
On a similar vein, a creation has no real blood ties to its creator when divinity is involved. Yes it's like a potter romancing their vase or an artist romancing their own OC but we're also operating on a very finite, mortal scale. Most all gods are written and understood in a much grander, timeless scale: if they do age, it's over thousands and millions of years. After a while I guess it just doesn't matter to them anymore. Zeus is an example of this, seeing as he'll stick his d*ck into any hole that catches his eye, even while said person has actual lineage to another demigod he sired. And as mentioned before, many founding myths link their ruler to divinity; in a way, you could say that anyone marrying into the royal family is already technically courting a deity.